Re: [Xen-devel] [PATCH v1 09/47] vidoe: fbdev: atyfb: remove and fix MTRR MMIO "hole" work around

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Wed, Apr 01, 2015 at 05:04:08PM -0700, Andy Lutomirski wrote:
> On Wed, Apr 1, 2015 at 4:52 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> > On Sat, Mar 28, 2015 at 02:23:34PM +0200, Ville Syrjälä wrote:
> >> On Sat, Mar 28, 2015 at 01:28:18AM +0100, Luis R. Rodriguez wrote:
> >> > On Fri, Mar 27, 2015 at 03:02:10PM -0700, Andy Lutomirski wrote:
> >> > > On Fri, Mar 27, 2015 at 2:56 PM, Ville Syrjälä <syrjala@xxxxxx> wrote:
> >> > > > On Fri, Mar 27, 2015 at 08:57:59PM +0100, Luis R. Rodriguez wrote:
> >> > > >> On Fri, Mar 27, 2015 at 12:43:55PM -0700, Andy Lutomirski wrote:
> >> > > >> > On Fri, Mar 27, 2015 at 12:38 PM, Luis R. Rodriguez 
> >> > > >> > <mcgrof@xxxxxxxx> wrote:
> >> > > >> > > On Sat, Mar 21, 2015 at 11:15:14AM +0200, Ville Syrjälä wrote:
> >> > > >> > >> On Fri, Mar 20, 2015 at 04:17:59PM -0700, Luis R. Rodriguez 
> >> > > >> > >> wrote:
> >> > > >> > >> > diff --git a/drivers/video/fbdev/aty/atyfb_base.c 
> >> > > >> > >> > b/drivers/video/fbdev/aty/atyfb_base.c
> >> > > >> > >> > index 8025624..8875e56 100644
> >> > > >> > >> > --- a/drivers/video/fbdev/aty/atyfb_base.c
> >> > > >> > >> > +++ b/drivers/video/fbdev/aty/atyfb_base.c
> >> > > >> > >> > @@ -2630,21 +2630,10 @@ static int aty_init(struct fb_info 
> >> > > >> > >> > *info)
> >> > > >> > >> >
> >> > > >> > >> >  #ifdef CONFIG_MTRR
> >> > > >> > >> >     par->mtrr_aper = -1;
> >> > > >> > >> > -   par->mtrr_reg = -1;
> >> > > >> > >> >     if (!nomtrr) {
> >> > > >> > >> > -           /* Cover the whole resource. */
> >> > > >> > >> > -           par->mtrr_aper = mtrr_add(par->res_start, 
> >> > > >> > >> > par->res_size,
> >> > > >> > >> > +           par->mtrr_aper = mtrr_add(info->fix.smem_start,
> >> > > >> > >> > +                                     info->fix.smem_len,
> >> > > >> > >> >                                       MTRR_TYPE_WRCOMB, 1);
> >> > > >> > >>
> >> > > >> > >> MTRRs need power of two size, so how is this supposed to work?
> >> > > >> > >
> >> > > >> > > As per mtrr_add_page() [0] the base and size are just supposed 
> >> > > >> > > to be in units
> >> > > >> > > of 4 KiB, although the practice is to use powers of 2 in *some* 
> >> > > >> > > drivers this
> >> > > >> > > is not standardized and by no means recorded as a requirement. 
> >> > > >> > > Obviously
> >> > > >> > > powers of 2 will work too and you'd end up neatly aligned as 
> >> > > >> > > well. mtrr_add()
> >> > > >> > > will use mtrr_check() to verify the the same requirement. 
> >> > > >> > > Furthermore,
> >> > > >> > > as per my commit log message:
> >> > > >> >
> >> > > >> > Whatever the code may or may not do, the x86 architecture uses
> >> > > >> > power-of-two MTRR sizes.  So I'm confused.
> >> > > >>
> >> > > >> There should be no confusion, I simply did not know that *was* the
> >> > > >> requirement for x86, if that is the case we should add a check for 
> >> > > >> that
> >> > > >> and perhaps generalize a helper that does the power of two helper 
> >> > > >> changes,
> >> > > >> the cleanest I found was the vesafb driver solution.
> >> > > >>
> >> > > >> Thoughts?
> >> > > >
> >> > > > The vesafb solution is bad since you'll only end up covering only
> >> > > > the first 4MB of the framebuffer instead of the almost 8MB you want.
> >> > > > Which in practice will mean throwing away half the VRAM since you 
> >> > > > really
> >> > > > don't want the massive performance hit from accessing it as UC. And 
> >> > > > that
> >> > > > would mean giving up decent display resolutions as well :(
> >> > > >
> >> > > > And the other option of trying to cover the remainder with multiple 
> >> > > > ever
> >> > > > smaller MTRRs doesn't work either since you'll run out of MTRRs very
> >> > > > quickly.
> >> > > >
> >> > > > This is precisely why I used the hole method in atyfb in the first
> >> > > > place.
> >> > > >
> >> > > > I don't really like the idea of any new mtrr code not supporting that
> >> > > > use case, especially as these things tend to be present in older 
> >> > > > machines
> >> > > > where PAT isn't an option.
> >> > >
> >> > > According to the Intel SDM, volume 3, section 11.5.2.1, table 11-6,
> >> > > non-PAT CPUs that have a WC MTRR, PCD = 1, and PWT = 1 (aka UC) have
> >> > > an effective memory type of UC.
> >
> > This is true but non-PAT systems that use just ioremap() will default to
> > _PAGE_CACHE_MODE_UC_MINUS, not _PAGE_CACHE_MODE_UC, and 
> > _PAGE_CACHE_MODE_UC_MINUS
> > on Linux has PCD = 1, PWT = 0. The list comes from:
> >
> > uint16_t __cachemode2pte_tbl[_PAGE_CACHE_MODE_NUM] = {
> >         [_PAGE_CACHE_MODE_WB      ]     = 0         | 0        ,
> >         [_PAGE_CACHE_MODE_WC      ]     = _PAGE_PWT | 0        ,
> >         [_PAGE_CACHE_MODE_UC_MINUS]     = 0         | _PAGE_PCD,
> >         [_PAGE_CACHE_MODE_UC      ]     = _PAGE_PWT | _PAGE_PCD,
> >         [_PAGE_CACHE_MODE_WT      ]     = 0         | _PAGE_PCD,
> >         [_PAGE_CACHE_MODE_WP      ]     = 0         | _PAGE_PCD,
> > };
> >
> > This can better be read here:
> >
> >  PAT
> >  |PCD
> >  ||PWT
> >  |||
> >  000 WB          _PAGE_CACHE_MODE_WB
> >  001 WC          _PAGE_CACHE_MODE_WC
> >  010 UC-         _PAGE_CACHE_MODE_UC_MINUS
> >  011 UC          _PAGE_CACHE_MODE_UC
> >
> > On x86 ioremap() defaults to ioremap_nocache() and right now that uses
> > _PAGE_CACHE_MODE_UC_MINUS not _PAGE_CACHE_MODE_UC. We have two cases
> > to consider for non-PAT systems then:
> >
> > a) Right now as ioremap() and ioremap_nocache() default to 
> > _PAGE_CACHE_MODE_UC_MINUS
> >    on x86. In this case using a WC MTRR seems to use PWT=0, PCD=1, and
> >    table table 11-6 on non-PAT systems seems to place this situation as
> >    "implementation defined" and not encouraged.
> >
> > a) when commit de33c442e "x86 PAT: fix performance drop for glx, use
> >    UC minus for ioremap(), ioremap_nocache() and pci_mmap_page_range()"
> >    gets reverted and we use _PAGE_CACHE_MODE_UC by default. In this
> >    case on x86 for both ioremap() and ioremap_nocache() as they will
> >    both default to _PAGE_CACHE_MODE_UC we'll end up as you note with
> >    an effective memory type of UC.
> >
> > If I've understood this correctly then neither of these situations are good 
> > and
> > its just by chance that on some systems situation a) has lead to proper WC.
> >
> > On a PAT system we have a bit different combinatorial results (based on 
> > Table
> > 11-7):
> >
> > a) Right now ioremap() and ioremap_nocache() defaulting to
> >     _PAGE_CACHE_MODE_UC_MINUS yields + MTRR WC = WC
> >
> > b) When commit de33c442e gets reverted _PAGE_CACHE_MODE_UC + MTRR WC = UC
> >
> > So to be clear right now atyfb should work fine on PAT systems
> > with de33c442e in place, once reverted as-is right now we'd end
> > up with UC effective memory type.
> >
> > For both PAT and non-PAT systems when commit de33c442e gets reverted
> > we'd end up with UC as the effective memory type for atyfb. Right
> > now it shoud work on PAT systems and by chance its suspected to work
> > on non-PAT systems. We want to phase MTRR though, specially to avoid
> > all this insane combinatorial nightmware.
> >
> >> > > Hence my suggestion to add
> >> > > ioremap_x86_uc and/or set_memory_x86_uc to punch a UC hole in an
> >> > > otherwise WC MTRR-covered region.
> >
> > To be clear I think you mean then that ioremap_x86_uc() would help us avoid 
> > the
> > jumps between combinatorial issues with MTRR on PAT / non-PAT systems before
> > and after commit de33c442e gets reverted. So for instance if we had on the
> > atyfb driver:
> >
> > ioremap_x86_uc(PCI BAR)
> > ioremap_wc(framebuffer)
> > arch_phys_add_wc(PCI BAR)
> >
> > On non-PAT systems on the MMIO region with PWT=1, PCD=1 we'd end up with UC.
> > Sadly though since _PAGE_CACHE_WC on non-PAT has PWT=1, PCD=0, the WC
> > MTRR that follows would mean we'd end up with another grey area (but
> > similar to before as technically an effectivethe memory type of WC).
> >
> > On PAT systems the above would not use MTRRs but we'd be counting on
> > overlapping memory types -- its not clear if aliasing here is a problem.
> >
> > Also Intel SDM, volume 3, section "11.11.4 Range Size and Alignment 
> > Requirement"
> > describes that: "the minimum range size is 4 KiB, the base address must be 
> > on
> > a 4 KiB boundary. For ranges greater than 4 KiB each range must be of length
> > 2^n and its base address must be alinged on a 2^n boundary where n is a 
> > value
> > equal or greatar then 12. The base-address alignment value cannot be less
> > than its length. For example, an 8-KiB range cannot be aligned on a
> > 4-KiB boundary. It must be aligned on at least an 8-KiB boundary"
> >
> > So to answer my own question: indeed, our framebuffer base address must be
> > aligned on a 2^n boundary, the size also has to be a power of 2. MTRR 
> > supports
> > fixed range sizes and variable range sizes, in case of the MMIO that does
> > not need to abide by the power of 2 rule as a fixed range size of 4 KiB
> > could be used although upon review ouf our own implemetnation its unclear if
> > that is what is used for 4 KiB sized MTRRs.
> >
> > Hence my arch_phys_add_wc(PCI BAR) as above.
> >
> >> > OK I think I get it now.
> >> >
> >> > And I take it this would hopefully only be used for non-PAT systems?
> >
> > Since we likely could care to use ioremap_x86_uc() on PAT systems as well we
> > could make the effective for both PAT and non-PAT obviously then.  Later 
> > when
> > we get ioremap() to default to strong UC we could drop ioremap_x86_uc() as 
> > we'd
> > only need it as transitory until then -- that is unless we want perhaps a 
> > strong
> > UC ioremap primitive which is always following strong UC when available 
> > regardless
> > of these default transitions.
> >
> > The big issue I see here is simply the combinatorial issues, so I do think
> > its best to annotate these corner cases well and avoid them.
> >
> >> > Would there be a use case for PAT systems? I wonder if we can wrap
> >> > this under some APIs to make it clean and hide this dirty thing
> >> > behind the scenes, it seems a fragile and error prone and my hope
> >> > would be that we won't need more specialization in this area for
> >> > PAT systems.
> >>
> >> One potential complication is kernel vs. userspace mmap. MTRR applies to
> >> the physical address, but PAT applies to the virtual address, so with
> >> the WC MTRR you get WC for userspace "for free" as well.
> >
> > What is the performance impact of having the conversion being done by the
> > kernel? Has anyone done measurements? If significant can't the subsystem 
> > mmap()
> > cache the phys address for PAT? Shouldn't the TLB take care of those 
> > considerations
> > for us? If this is generally desirable shouldn't we just generalize the 
> > cache
> > for devices for O(1) access through a generic API?
> 
> We're pretty much required to keep the PTE memory types consistent for
> aliasses of the same page. 

Hrm, OK so overlapping ioremap() calls should be frowed upon?

I think its important to clarify the few different scenarios we have
for atyfb, both for today when uc- is default and when uc becomes the
default. I'll also clarify what this series originally tried to do
but the issues that size requirements prohibit us to do along with
combinatorial issues that would also be present when and if uc becomes
default. Finally I'll clarify what I am thinking we should do in light
of all this.

_______________________________________________________________________
|                                                       |             |
|_______________________________________________________|_____________|

\______________________________________________________/ \____________/

                Framebuffer (8 MiB)                         MMIO (4 KiB)

Currently we have:

Page_cache_mode's _PAGE_CACHE_MODE_ is removed below for brevity.
The atyfb PCI BAR is condensed to:

Frambuffer,MMIO

Keeping in mind:

Intel SDM, volume 3, section 11.5.2.1, table 11-6 (NonPAT combinatorial)
Intel SDM, volume 3, section 11.5.2.2, table 11-7 (PAT    combinatorial)

Linux PCD, PWT bits:

 PAT
 |PCD
 ||PWT
 |||
 000 WB          _PAGE_CACHE_MODE_WB
 001 WC          _PAGE_CACHE_MODE_WC
 010 UC-         _PAGE_CACHE_MODE_UC_MINUS
 011 UC          _PAGE_CACHE_MODE_UC

(*)   below denotes grey area as per SDM, implementation-defined
(%)   below denotes not posislbe due to size / base requirements of MTRRs
(+)   below denotes combinatorial issue

Non-PAT systems use PCD, PWT values, their respective bit settings for
these are given although internally we use _PAGE_CACHE_MODE* on the
ioremap* calls for both non-PAT and PAT. For instance
_PAGE_CACHE_MODE_UC_MINUS is 10 for PCD=1, PWT=0.

Today we have:

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap(MMIO)           | xxx, 10  | xxx, UC- | xxx, UC  | xxx, UC- |
ioremap(PCI BAR)        | 10 , 10  | UC-, UC- | UC,  UC  | UC-, UC- |
MTRR WC(PCI BAR)        | 10 , 10  | UC-, UC- | WC*, WC* | WC , WC  |
MTRR UC(MMIO)           | 10 , 10  | UC-, UC- | WC*, UC  | WC , UC  |
--------------------------------------------------------------------

If today we revert commit de33c442e and UC becomes default this would run into
the combinatorial issue:

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap(MMIO)           | xxx, 11  | xxx, UC  | xxx, UC  | xxx, UC  |
ioremap(PCI BAR)        | 11 , 11  | UC , UC  | UC,  UC  | UC , UC  |
MTRR WC(PCI BAR)        | 11 , 11  | UC,  UC  | UC+, UC+ | UC+, UC+ |
MTRR UC(MMIO)           | 11 , 11  | UC,  UC  | UC+, UC  | UC+, UC  |
--------------------------------------------------------------------

We ideally would like to do the following but can't because of the restriction
of having to use powers of two for both size and base address for MTRRs, we'd
have two steps, one with mtrr_add, and another with arch_phys_add_wc(). This is
what this series was proposing for atyfb.

With mtrr_add():

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_nocache(MMIO)   | xxx, 10  | xxx, UC- | xxx, UC  | xxx, UC- |
ioremap_wc(fb)          | 01 , 10  | WC , UC- | UC , UC  | WC , UC- |
MTRR WC(fb)             | 01 , 10  | UC-, WC  | WC%*,UC  | WC%, UC- |
--------------------------------------------------------------------

Then we'd change this to arch_phys_add_wc():

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_nocache(MMIO)   | xxx, 10  | xxx, UC- | xxx, UC  | UC-, UC- |
ioremap_wc(fb)          | 01 , 10  | WC , UC- | UC , UC  | WC , UC- |
arch_phys_add_wc(fb)    | 01 , 10  | WC , WC  | WC%*,UC  | WC , UC- |
--------------------------------------------------------------------

With the above code as well we have to consider the issues if we
revert commit de33c442e and UC becomes default, we'd run into then
both the size issue and also a grey area:

With mtrr_add():

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_nocache(MMIO)   | xxx, 11  | xxx, UC  | xxx, UC  | xxx, UC  |
ioremap_wc(fb)          | 01 , 11  | WC , UC  | UC , UC  | WC , UC  |
MTRR WC(fb)             | 01 , 11  | WC , UC  | WC%* ,UC  | WC , UC  |
--------------------------------------------------------------------

Then with arch_phys_add_wc():

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_nocache(MMIO)   | xxx, 11  | xxx, UC  | xxx, UC  | xxx, UC  |
ioremap_wc(fb)          | 01 , 11  | WC , UC  | UC , UC  | WC , UC  |
arch_phys_add_wc(fb)    | 01 , 11  | WC , UC  | WC%*,UC  | WC , UC  |
--------------------------------------------------------------------

So what we *could* do then if we add ioremap_uc() (use strong UC always),
then override the framebuffer area with wc, and finally use MTRR on the
full PCI BAR, relying on that strong UC won't let the MTRR override
the earlier UC on the MMIO area. There is a grey area here for non-PAT
systemes but that is also the case as-is today.

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_uc(PCI BAR)     | 11 , 11  | UC , UC  | UC , UC  | UC , UC  |
ioremap_wc(fb)          | 01 , 11  | WC , UC  | UC , UC  | WC , UC  |
MTRR_WC(PCI BAR)        | 01 , 11  | WC , UC  | WC*, UC  | WC , UC  |
--------------------------------------------------------------------

Finally with the arch_phys_add_wc() we'd end up with:

--------------------------------------------------------------------
Calls                   |    Page_cache_mode  |  Effective memtype  |
------------------------|---------------------|---------------------
                        |  Non-PAT |    PAT   |  Non-PAT |    PAT   |
--------------------------------------------------------------------
ioremap_uc(PCI BAR)     | 11 , 11  | UC , UC  | UC , UC  | UC , UC  |
ioremap_wc(fb)          | 01 , 11  | WC , UC  | UC , UC  | WC , UC  |
arch_phys_add_wc(PCIBAR)| 01 , 11  | WC , UC  | WC*, UC  | WC , UC  |
--------------------------------------------------------------------

In this case a revert of de33c442e won't have any effect as the driver
was already well prepared for it by using ioremap_uc().

> I think that the x86 pageattr code is
> supposed to take care of this.  IOW, if everything is working right,
> then the supposedly uncached mmap should either fail, be promoted to
> WC, or cause the existing WC map to degrade to UC.  The code is really
> overcomplicated right now.

Yeah aliasing things are not clear for the above picture for me, someone
who is knee-deep in this can likely confirm of any issues with the above
pictures. But most importrantly if we believe however that the last two sets
above don't have any issues then I think we can move forward. Since we only
have a few drivers that need special handling I think it makes sense to treat
them specially and document this strategy for the "hole" work around.

Thoughts?

  Luis

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel