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Subject: Re: [PATCH 3/3 v4] KVM: SVM: Don't flush cache if hardware enforces
 cache coherency across encryption domains
To:     Borislav Petkov <bp@alien8.de>,
        Krish Sadhukhan <krish.sadhukhan@oracle.com>
Cc:     kvm@vger.kernel.org, jmattson@google.com, tglx@linutronix.de,
        mingo@redhat.com, x86@kernel.org, sean.j.christopherson@intel.com,
        vkuznets@redhat.com, wanpengli@tencent.com, joro@8bytes.org,
        dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org,
        linux-kernel@vger.kernel.org, hpa@zytor.com
References: <20200917212038.5090-1-krish.sadhukhan@oracle.com>
 <20200917212038.5090-4-krish.sadhukhan@oracle.com>
 <20200918075651.GC6585@zn.tnic>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <b711fc52-0adf-1afd-61c8-ce142bff9ddb@redhat.com>
Date:   Sat, 19 Sep 2020 17:18:58 +0200
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On 18/09/20 09:56, Borislav Petkov wrote:
> On Thu, Sep 17, 2020 at 09:20:38PM +0000, Krish Sadhukhan wrote:
>> In some hardware implementations, coherency between the encrypted and
>> unencrypted mappings of the same physical page in a VM is enforced. In such a
>> system, it is not required for software to flush the VM's page from all CPU
>> caches in the system prior to changing the value of the C-bit for the page.
>>
>> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
>> ---
>>  arch/x86/kvm/svm/sev.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 7bf7bf734979..3c9a45efdd4d 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -384,7 +384,8 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
>>  	uint8_t *page_virtual;
>>  	unsigned long i;
>>  
>> -	if (npages == 0 || pages == NULL)
>> +	if (this_cpu_has(X86_FEATURE_SME_COHERENT) || npages == 0 ||
>> +	    pages == NULL)
>>  		return;
>>  
>>  	for (i = 0; i < npages; i++) {
>> -- 
> 
> Took the first two, Paolo lemme know if I should route this one through
> tip too.
> 
> Thx.
> 

Yeah, it's innocuous enough as far as conflicts are concerned.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo