Re: [Xen-devel] Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup

[Toshi Kani <toshi.kani@xxxxxxx>]

On Fri, 2015-09-04 at 03:40 +0200, Luis R. Rodriguez wrote:
> On Thu, Sep 03, 2015 at 06:48:46PM -0600, Toshi Kani wrote:
> > On Fri, 2015-09-04 at 01:54 +0200, Luis R. Rodriguez wrote:
> > > On Thu, Sep 03, 2015 at 05:21:14PM -0600, Toshi Kani wrote:
> > > > On Fri, 2015-09-04 at 00:45 +0200, Luis R. Rodriguez wrote:
> > > > > On Thu, Sep 03, 2015 at 04:25:31PM -0600, Toshi Kani wrote:
> >  :
> > > > > > On Xen,
> > > > > 
> > > > > When Xen is used a platform firmware may still set up MTRR, even if 
> > > > > the hypervisor doesn't set up MTRR right ? So same issue and 
> > > > > question here.
> > > > 
> > > > Right, I meant to say Xen guests.
> > > 
> > > Ah but its import complicated than that.
> > > 
> > > > In case of the Xen hypervisor,
> > > > mtrr_type_lookup() returns a valid type as it runs on a platform.
> > > 
> > > I am not sure if this happens today, I know MTRR is simply disabled by
> > > the Xen Hypervisor on the CPU explicitly, it disable it so guests 
> > > reading the MTRR capabilities sees it as disabled when queried.
> > 
> > Oh, I would not let the hypervisor to disable MTRRs...
> 
> Commit 586ab6a055376ec3f3e1e8 ("x86/pvh: disable MTRR feature on cpuid for 
> Dom0") by Roger Pau Monnà disables MTRR for PVH dom0, so that cpuid returns 
> that MTRR is disabled to guests. 

Oh, I see.  It just clears the capability bit so that the kernel thinks MTRRs
are disabled.  That makes sense.

> Then later on Linux as of commit 47591df50512 ("xen: Support Xen pv-domains
> using PAT") added by Juergen as of v3.19 Linux guests can end up booting
> without MTRR but with PAT now enabled.

Nice!

> > > Then since the Xen Linux guests cannot speak MTRR through the hypervisor
> > > (for instance Xen guests cannot ask Xen hypervisor to mtrr_type_lookup() 
> > > for it) if PCI passthrough is used it could mean a guest might set up / 
> > > use incorrect info as well.
> > > 
> > > If I undestand this correctly then I think we're in a pickle with Xen 
> > > unless we add hypervisor support and hypercall support for
> > > mtrr_type_lookup().
> > 
> > I was under assumption that MTRRs are emulated and disabled on guests.
> 
> Some "special" flavor Linux guests (with non-upstream code) have guest
> MTRR hypercall support, for vanilla Xen and Linux they just never get MTRR
> support. After Juergen's Linux changes though Xen guests can now get
> shiny PAT support. Since MTRR hypercall support is not upstream and MTRR is
> ancient I decided instead of adding MTRR hypercall support upstream to go 
> with converting all drivers to PAT interfaces, with the assumption there 
> would be no issues.
> 
> > Isn't guest physical address virtualized?
> 
> It is, there is a xen iotlb and stuff but that should ensure dom0 gets
> to get proper access to devices, and if you use PCI passthrough you want
> the best experience as well.
> 
> > I know other proprietary VMMs on IA64, but know nothing about Xen...  So, 
> > please disregard my comments to Xen. :-)
> 
> No worries, no one knows all the answers, we work together to remove
> cob webs off of these odd corners no one cares about :)

Thanks for all the info!  That helps.
-Toshi

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