Messages in this thread | | | Date | Tue, 10 Jan 2023 06:47:57 +0100 | Subject | Re: Problem with pat_enable() and commit 72cbc8f04fe2 | From | Juergen Gross <> |
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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 [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |