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| author | Sean Christopherson <seanjc@google.com> | 2024-03-09 02:09:26 +0100 |
|---|---|---|
| committer | Sean Christopherson <seanjc@google.com> | 2024-06-05 17:13:14 +0200 |
| commit | e1548088ff544e4a4ae2c960bccf0eeff3a46297 (patch) | |
| tree | 44eed91ec9a6107f900f487e95288456fcd86e0b /Documentation/virt/kvm/x86/errata.rst | |
| parent | KVM: x86: Remove VMX support for virtualizing guest MTRR memtypes (diff) | |
| download | linux-e1548088ff544e4a4ae2c960bccf0eeff3a46297.tar.xz linux-e1548088ff544e4a4ae2c960bccf0eeff3a46297.zip | |
KVM: VMX: Drop support for forcing UC memory when guest CR0.CD=1
Drop KVM's emulation of CR0.CD=1 on Intel CPUs now that KVM no longer
honors guest MTRR memtypes, as forcing UC memory for VMs with
non-coherent DMA only makes sense if the guest is using something other
than PAT to configure the memtype for the DMA region.
Furthermore, KVM has forced WB memory for CR0.CD=1 since commit
fb279950ba02 ("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED"), and no known
VMM in existence disables KVM_X86_QUIRK_CD_NW_CLEARED, let alone does
so with non-coherent DMA.
Lastly, commit fb279950ba02 ("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED") was
from the same author as commit b18d5431acc7 ("KVM: x86: fix CR0.CD
virtualization"), and followed by a mere month. I.e. forcing UC memory
was likely the result of code inspection or perhaps misdiagnosed failures,
and not the necessitate by a concrete use case.
Update KVM's documentation to note that KVM_X86_QUIRK_CD_NW_CLEARED is now
AMD-only, and to take an erratum for lack of CR0.CD virtualization on
Intel.
Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Link: https://lore.kernel.org/r/20240309010929.1403984-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Diffstat (limited to 'Documentation/virt/kvm/x86/errata.rst')
| -rw-r--r-- | Documentation/virt/kvm/x86/errata.rst | 19 |
1 files changed, 15 insertions, 4 deletions
diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst index 1b70bad7325e..4116045a8744 100644 --- a/Documentation/virt/kvm/x86/errata.rst +++ b/Documentation/virt/kvm/x86/errata.rst @@ -51,7 +51,18 @@ matching the target APIC ID receive the interrupt). MTRRs ----- -KVM does not virtualization guest MTRR memory types. KVM emulates accesses to -MTRR MSRs, i.e. {RD,WR}MSR in the guest will behave as expected, but KVM does -not honor guest MTRRs when determining the effective memory type, and instead -treats all of guest memory as having Writeback (WB) MTRRs.
\ No newline at end of file +KVM does not virtualize guest MTRR memory types. KVM emulates accesses to MTRR +MSRs, i.e. {RD,WR}MSR in the guest will behave as expected, but KVM does not +honor guest MTRRs when determining the effective memory type, and instead +treats all of guest memory as having Writeback (WB) MTRRs. + +CR0.CD +------ +KVM does not virtualize CR0.CD on Intel CPUs. Similar to MTRR MSRs, KVM +emulates CR0.CD accesses so that loads and stores from/to CR0 behave as +expected, but setting CR0.CD=1 has no impact on the cachaeability of guest +memory. + +Note, this erratum does not affect AMD CPUs, which fully virtualize CR0.CD in +hardware, i.e. put the CPU caches into "no fill" mode when CR0.CD=1, even when +running in the guest.
\ No newline at end of file |