linux-kernel - re: Possible 5.19 regression for systems with 52-bit physical address support

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Date:   Thu, 28 Jul 2022 08:44:30 -0500
From:   Michael Roth <michael.roth@....com>
To:     <seanjc@...gle.com>
CC:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: re: Possible 5.19 regression for systems with 52-bit physical
 address support

Hi Sean,

With this patch applied, AMD processors that support 52-bit physical
address will result in MMIO caching being disabled. This ends up
breaking SEV-ES and SNP, since they rely on the MMIO reserved bit to
generate the appropriate NAE MMIO exit event.

This failure can also be reproduced on Milan by disabling mmio_caching
via KVM module parameter.

In the case of AMD, guests use a separate physical address range that
and so there are still reserved bits available to make use of the MMIO
caching. This adjustment happens in svm_adjust_mmio_mask(), but since
mmio_caching_enabled flag is 0, any attempts to update masks get
ignored by kvm_mmu_set_mmio_spte_mask().

Would adding 'force' parameter to kvm_mmu_set_mmio_spte_mask() that
svm_adjust_mmio_mask() can set to ignore enable_mmio_caching be
reasonable fix, or should we take a different approach?

Thanks!

-Mike