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

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Message-ID: <559D0754.6030009@redhat.com>
Date:	Wed, 8 Jul 2015 13:19:48 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	jroedel@...e.de, alex.williamson@...hat.com, ogerlitz@...lanox.com,
	amirv@...lanox.com
Subject: Re: [PATCH 2/4] KVM: SVM: use NPT page attributes



On 08/07/2015 07:59, Xiao Guangrong wrote:
> 
> 
> 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@...hat.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

Oops, you're right.  In fact my first version was correct, then I
changed it to use cachemode2protval and screwed up.

Paolo
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