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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.06.2024 18:21, Roger Pau Monné wrote:
> On Tue, Jun 11, 2024 at 04:53:22PM +0200, Jan Beulich wrote:
>> On 11.06.2024 15:52, Roger Pau Monné wrote:
>>> On Tue, Jun 11, 2024 at 01:52:58PM +0200, Jan Beulich wrote:
>>>> On 11.06.2024 13:08, Roger Pau Monné wrote:
>>>>> I really wonder whether Xen has enough information to figure out
>>>>> whether a hole (MMIO region) is supposed to be accessed as UC or
>>>>> something else.
>>>>
>>>> It certainly hasn't, and hence is erring on the (safe) side of forcing
>>>> UC.
>>>
>>> Except that for the vesa framebuffer at least this is a bad choice :).
>>
>> Well, yes, that's where we want WC to be permitted. But for that we only
>> need to avoid setting iPAT; we still can uniformly hand back UC. Except
>> (as mentioned elsewhere earlier) if the guest uses MTRRs rather than PAT
>> to arrange for WC.
> 
> If we want to get this into 4.19, we likely want to go your proposed
> approach then, as it's less risky.
> 
> I think a comment would be helpful to note that the fix here to not
> enforce iPAT by still return UC is mostly done to accommodate vesa
> regions mapped with PAT attributes to use WC.
> 
> I would also like to add some kind of note that special casing
> !mfn_valid() might not be needed, but that removing it must be done
> carefully to not cause regressions.

Hmm, in the meantime I have myself sufficiently convinced that with a
small (hopefully easy / uncontroversial) change to ept_set_entry() I
can arrange for having the guarantees that neither INVALID_MFN nor a
truncated for of it can make it into the function, allowing the check
to be dropped (as you had initially asked for).

>>>> One caveat here that I forgot to
>>>> mention before: MFNs taken out of EPT entries will never be INVALID_MFN, 
>>>> for
>>>> the truncation that happens when populating entries. In that case we rely 
>>>> on
>>>> mfn_valid() to be "rejecting" them.
>>>
>>> The only caller where mfns from EPT entries are passed to
>>> epte_get_entry_emt() is in resolve_misconfig() AFAICT, and in that
>>> case the EPT entry must be present for epte_get_entry_emt() to be
>>> called.  So it seems to me that epte_get_entry_emt() can never be
>>> called from an mfn constructed from an INVALID_MFN EPT entry (but it's
>>> worth adding an assert for it).
>>
>> Are you sure? I agree for the first of those two calls, but the second
>> isn't quite as obvious. There we'd need to first prove that we will
>> never create non-present super-page entries. Yet if I'm not mistaken
>> for PoD we may create such.
> 
> I should go look then, didn't know PoD would do that.

I've meanwhile checked, and indeed we do. That's what with said prereq
change I hope to make no longer be the case.

Jan