@@ -7946,7 +7946,11 @@ The valid bits in cap.args[0] are:
When this quirk is disabled, the reset value
is 0x10000 (APIC_LVT_MASKED).
- KVM_X86_QUIRK_CD_NW_CLEARED By default, KVM clears CR0.CD and CR0.NW.
+ KVM_X86_QUIRK_CD_NW_CLEARED By default, KVM clears CR0.CD and CR0.NW on
+ AMD CPUs to workaround buggy guest firmware
+ that runs in perpetuity with CR0.CD, i.e.
+ with caches in "no fill" mode.
+
When this quirk is disabled, KVM does not
change the value of CR0.CD and CR0.NW.
@@ -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
@@ -7612,10 +7612,6 @@ static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;
- if (kvm_read_cr0_bits(vcpu, X86_CR0_CD) &&
- !kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
- return (MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;
-
return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT);
}
@@ -960,12 +960,6 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS)
kvm_mmu_reset_context(vcpu);
-
- if (((cr0 ^ old_cr0) & X86_CR0_CD) &&
- kvm_mmu_may_ignore_guest_pat() &&
- kvm_arch_has_noncoherent_dma(vcpu->kvm) &&
- !kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
- kvm_zap_gfn_range(vcpu->kvm, 0, ~0ULL);
}
EXPORT_SYMBOL_GPL(kvm_post_set_cr0);
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. Signed-off-by: Sean Christopherson <seanjc@google.com> --- Documentation/virt/kvm/api.rst | 6 +++++- Documentation/virt/kvm/x86/errata.rst | 19 +++++++++++++++---- arch/x86/kvm/vmx/vmx.c | 4 ---- arch/x86/kvm/x86.c | 6 ------ 4 files changed, 20 insertions(+), 15 deletions(-)