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
fb279950 ("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 fb279950 ("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED") was
from the same author as commit b18d5431 ("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.comSigned-off-by: Sean Christopherson <seanjc@google.com>

Documentation/virt/kvm/api.rst

@@ -7953,7 +7953,11 @@ 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.
 

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

arch/x86/kvm/vmx/vmx.c

@@ -7674,10 +7674,6 @@ 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);
 }
 

arch/x86/kvm/x86.c

@@ -963,12 +963,6 @@ 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);