Re: [PATCH] x86/EPT: relax iPAT for "invalid" MFNs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.06.2024 11:02, Roger Pau Monné wrote:
> On Tue, Jun 11, 2024 at 10:26:32AM +0200, Jan Beulich wrote:
>> On 11.06.2024 09:41, Roger Pau Monné wrote:
>>> On Mon, Jun 10, 2024 at 04:58:52PM +0200, Jan Beulich wrote:
>>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>>> @@ -503,7 +503,8 @@ int epte_get_entry_emt(struct domain *d,
>>>>  
>>>>      if ( !mfn_valid(mfn) )
>>>>      {
>>>> -        *ipat = true;
>>>> +        *ipat = type != p2m_mmio_direct ||
>>>> +                (!is_iommu_enabled(d) && !cache_flush_permitted(d));
>>>
>>> Looking at this, shouldn't the !mfn_valid special case be removed, and
>>> mfns without a valid page be processed normally, so that the guest
>>> MTRR values are taken into account, and no iPAT is enforced?
>>
>> Such removal is what, in the post commit message remark, I'm referring to
>> as "moving to too lax". Doing so might be okay, but will imo be hard to
>> prove to be correct for all possible cases. Along these lines goes also
>> that I'm adding the IOMMU-enabled and cache-flush checks: In principle
>> p2m_mmio_direct should not be used when neither of these return true. Yet
>> a similar consideration would apply to the immediately subsequent if().
>>
>> Removing this code would, in particular, result in INVALID_MFN getting a
>> type of WB by way of the subsequent if(), unless the type there would
>> also be p2m_mmio_direct (which, as said, it ought to never be for non-
>> pass-through domains). That again _may_ not be a problem as long as such
>> EPT entries would never be marked present, yet that's again difficult to
>> prove.
> 
> My understanding is that the !mfn_valid() check was a way to detect
> MMIO regions in order to exit early and set those to UC.  I however
> don't follow why the guest MTRR settings shouldn't also be applied to
> those regions.

It's unclear to me whether the original purpose of he check really was
(just) MMIO. It could as well also have been to cover the (then not yet
named that way) case of INVALID_MFN.

As to ignoring guest MTRRs for MMIO: I think that's to be on the safe
side. We don't want guests to map uncachable memory with a cachable
memory type. Yet control isn't fine grained enough to prevent just
that. Hence why we force UC, allowing merely to move to WC via PAT.

> I'm also confused by your comment about "as such EPT entries would
> never be marked present": non-present EPT entries don't even get into
> epte_get_entry_emt(), and hence we could assert in epte_get_entry_emt
> that mfn != INVALID_MFN?

I don't think we can. Especially for the call from ept_set_entry() I
can't spot anything that would prevent the call for non-present entries.
This may be a mistake, but I can't do anything about it right here.

>> I was in fact wondering whether to special-case INVALID_MFN in the change
>> I'm making. Question there is: Are we sure that by now we've indeed got
>> rid of all arithmetic mistakenly done on MFN variables happening to hold
>> INVALID_MFN as the value? IOW I fear that there might be code left which
>> would pass in INVALID_MFN masked down to a 2M or 1G boundary. At which
>> point checking for just INVALID_MFN would end up insufficient. If we
>> meant to rely on this (tagging possible leftover issues as bugs we don't
>> mean to attempt to cover for here anymore), then indeed the mfn_valid()
>> check could be replaced by a comparison with INVALID_MFN (following a
>> pattern we've been slowly trying to carry through elsewhere, especially
>> in shadow code). Yet it could still not be outright dropped imo.
>>
>> Furthermore simply dropping (or replacing as per above) that check won't
>> work either: Further down in the function we use mfn_to_page(), which
>> requires an up-front mfn_valid() check. That said, this code looks
>> partly broken to me anyway: For a 1G page mfn_valid() on the start of it
>> doesn't really imply all parts of it are valid. I guess I need to make a
>> 2nd patch to address that as well, which may then want to be a prereq
>> change to the one here (if we decided to go the route you're asking for).
> 
> I see, yes, the loop over the special pages array will need to be
> adjusted to account for mfn_to_page() possibly returning NULL.

Except that NULL will hardly ever come back there. What we need is an
explicit mfn_valid() check. I already have a patch, but I'd like to
submit it only once I know how the v2 of the one here is going to look
like.

> Overall I don't understand the need for this special case for
> !mfn_valid().  The rest of special cases we have (the special pages
> and domains without devices or MMIO regions assigned) are performance
> optimizations which I do understand.  Yet the special casing of
> !mfn_valid regions bypassing guest MTRR settings seems bogus to me.

As said, it may well be that we can (now) switch to comparison against
INVALID_MFN there, if we're certain MMIO isn't to be covered by this
(anymore).

>>> I also think this likely wants a:
>>>
>>> Fixes: 81fd0d3ca4b2 ('x86/hvm: simplify 'mmio_direct' check in 
>>> epte_get_entry_emt()')
>>
>> Oh, indeed, I should have dug out when this broke. I didn't because I
>> knew this mfn_valid() check was there forever, neglecting that it wasn't
>> always (almost) first.
>>
>>> As AFAICT before that commit direct MMIO regions would set iPAT to WB,
>>> which would result in the correct attributes (albeit guest MTRR was
>>> still ignored).
>>
>> Two corrections here: First iPAT is a boolean; it can't be set to WB.
>> And then what was happening prior to that change was that for the APIC
>> access page iPAT was set to true, thus forcing WB there. iPAT was left
>> set to false for all other p2m_mmio_direct pages, yielding (PAT-
>> overridable) UC there.
> 
> Right, that behavior was still dubious to me, as I would assume those
> regions would also want to fetch the type from guest MTRRs.

Well, for the APIC access page we want to prevent it becoming UC. It's MMIO
from the guest's perspective, yet _we_ know it's really ordinary RAM. For
actual MMIO see above; the only case where we probably ought to respect
guest MTRRs is when they say WC (following from what I said further up).
Yet that's again an independent change to (possibly) make.

Jan