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Re: Problem with pat_enable() and commit 72cbc8f04fe2



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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