linux-kernel - Re: [PATCH Part2 RFC v4 20/40] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

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Date:   Tue, 3 Aug 2021 09:38:57 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 20/40] KVM: SVM: Make AVIC backing, VMSA and
 VMCB memory allocation SNP safe

Hi Sean,


On 7/20/21 1:02 PM, Sean Christopherson wrote:
> IMO, the CPU behavior is a bug, even if the behavior is working as intended for
> the microarchitecture.  I.e. this should be treated as an erratum.
> 

I agreed with your comment that it should be treated as an erratum. I 
now have agreement from the hardware team to publish this as an erratum 
with explanation and recommendation. This will certainly help in 
documenting on "why" we are making the page split.

...

>>   
>> -	apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
>> +	if (kvm_x86_ops.alloc_apic_backing_page)
>> +		apic->regs = kvm_x86_ops.alloc_apic_backing_page(vcpu);
> 
> This can be a static_call().

Noted.

> 
> This isn't "finding" anything, it's identifying which of the two pages is
> _guaranteed_ to be unaligned.  The whole function needs a much bigger comment to
> explain what's going on.

Let me add more comment to clarify it.

> 
>> +	pfn = page_to_pfn(p);
>> +	if (IS_ALIGNED(__pfn_to_phys(pfn), PMD_SIZE)) {
>> +		pfn++;
>> +		__free_page(p);
>> +	} else {
>> +		__free_page(pfn_to_page(pfn + 1));
>> +	}
>> +
>> +	return pfn_to_page(pfn);
>> +}

thanks