Re: [PATCH 2/4] KVM: SVM: use NPT page attributes

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 07/07/2015 09:45 PM, Paolo Bonzini wrote:
> Right now, NPT page attributes are not used, and the final page
> attribute depends solely on gPAT (which however is not synced
> correctly), the guest MTRRs and the guest page attributes.
>
> However, we can do better by mimicking what is done for VMX.
> In the absence of PCI passthrough, the guest PAT can be ignored
> and the page attributes can be just WB.  If passthrough is being
> used, instead, keep respecting the guest PAT, and emulate the guest
> MTRRs through the PAT field of the nested page tables.
>
> The only snag is that WP memory cannot be emulated correctly,
> because Linux's default PAT setting only includes the other types.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   arch/x86/kvm/svm.c | 47 ++++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 42 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 602b974a60a6..0f125c1860ec 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1085,6 +1085,47 @@ static u64 svm_compute_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
>   	return target_tsc - tsc;
>   }
>
> +static void svm_set_guest_pat(struct vcpu_svm *svm, u64 *g_pat)
> +{
> +	struct kvm_vcpu *vcpu = &svm->vcpu;
> +
> +	/* Unlike Intel, AMD takes the guest's CR0.CD into account.

I noticed this code in svm_set_cr0():

	if (!(vcpu->kvm->arch.disabled_quirks & KVM_QUIRK_CD_NW_CLEARED))
		cr0 &= ~(X86_CR0_CD | X86_CR0_NW);

gCR0.CD is hidden to CPU if KVM_QUIRK_CD_NW_CLEARED is not set and looks like
it is the normal case after grepping Qemu code.

> +	 *
> +	 * AMD does not have IPAT.  To emulate it for the case of guests
> +	 * with no assigned devices, just set everything to WB.  If guests
> +	 * have assigned devices, however, we cannot force WB for RAM
> +	 * pages only, so use the guest IPAT as passed.
> +	 */
> +	if (!kvm_arch_has_assigned_device(vcpu->kvm))
> +		*g_pat = 0x0606060606060606;
> +	else
> +		*g_pat = vcpu->arch.pat;
> +}
> +
> +static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> +{
> +	u8 cache;
> +
> +	/*
> +	 * 1. MMIO: always map as UC
> +	 * 2. No passthrough: always map as WB, and force guest PAT to WB as well
> +	 * 3. Passthrough: can't guarantee the result, try to trust guest.
> +	 */
> +	if (is_mmio)
> +		return _PAGE_NOCACHE;
> +
> +	if (!kvm_arch_has_assigned_device(vcpu->kvm))
> +		return 0;
> +
> +	cache = kvm_mtrr_get_guest_memory_type(vcpu, gfn);
> +

@cache return from kvm_mtrr_get_guest_memory_type is MTRR_TYPE_*
which is different with _PAGE_CACHE_MODE_*. The latter is pure SW
usage, e.g:
_PAGE_CACHE_MODE_WB = 0 and  #define MTRR_TYPE_WRBACK     6