Messages in this thread | | | Date | Tue, 10 Jan 2023 06:59:37 +0100 | Subject | Re: Problem with pat_enable() and commit 72cbc8f04fe2 | From | Juergen Gross <> |
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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 [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |