Re: Problem with pat_enable() and commit 72cbc8f04fe2

[Juergen Gross <jgross@xxxxxxxx>]

On 10.01.23 06:47, Juergen Gross wrote:
> 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.
Another idea would be to let the mtrr_type_lookup() stub in
arch/x86/include/asm/mtrr.h return MTRR_TYPE_WRBACK, enabling to simplify
pud_set_huge() and pmd_set_huge() by removing the check for MTRR_TYPE_INVALID.


Juergen

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