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SubjectRe: Problem with pat_enable() and commit 72cbc8f04fe2
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On 09.01.23 19:28, Michael Kelley (LINUX) wrote:
> I've come across a case with a VM running on Hyper-V that doesn't get
> MTRRs, but the PAT is functional. (This is a Confidential VM using
> AMD's SEV-SNP encryption technology with the vTOM option.) In this
> case, the changes in commit 72cbc8f04fe2 ("x86/PAT: Have pat_enabled()
> properly reflect state when running on Xen") apply. pat_enabled() returns
> "true", but the MTRRs are not enabled.
>
> But with this commit, there's a problem. Consider memremap() on a RAM
> region, called with MEMREMAP_WB plus MEMREMAP_DEC as the 3rd
> argument. Because of the request for a decrypted mapping,
> arch_memremap_can_ram_remap() returns false, and a new mapping
> must be created, which is appropriate.
>
> The following call stack results:
>
> memremap()
> arch_memremap_wb()
> ioremap_cache()
> __ioremap_caller()
> memtype_reserve() <--- pcm is _PAGE_CACHE_MODE_WB
> pat_x_mtrr_type() <-- only called after commit 72cbc8f04fe2
>
> pat_x_mtrr_type() returns _PAGE_CACHE_MODE_UC_MINUS because
> mtrr_type_lookup() fails. As a result, memremap() erroneously creates the
> new mapping as uncached. This uncached mapping is causing a significant
> performance problem in certain Hyper-V Confidential VM configurations.
>
> Any thoughts on resolving this? Should memtype_reserve() be checking
> both pat_enabled() *and* whether MTRRs are enabled before calling
> pat_x_mtrr_type()? Or does that defeat the purpose of commit
> 72cbc8f04fe2 in the Xen environment?

I think pat_x_mtrr_type() should return _PAGE_CACHE_MODE_UC_MINUS only if
mtrr_type_lookup() is not failing and is returning a mode other than WB.

I'll send a patch.

> I'm also looking at how to avoid this combination in a Hyper-V Confidential
> VM, but that doesn't address underlying the flaw.

Yes.


Juergen
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