linux-kernel - Re: [PATCH v1 2/2] x86/sev: Add SNP-specific unaccepted memory support

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Date:   Tue, 23 Aug 2022 09:28:01 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dionna Amalie Glaze <dionnaglaze@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1 2/2] x86/sev: Add SNP-specific unaccepted memory
 support

On 8/22/22 19:24, Dionna Amalie Glaze wrote:
>>
>> +void snp_accept_memory(phys_addr_t start, phys_addr_t end)
>> +{
>> +       unsigned long vaddr;
>> +       unsigned int npages;
>> +
>> +       if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
>> +               return;
>> +
>> +       vaddr = (unsigned long)__va(start);
>> +       npages = (end - start) >> PAGE_SHIFT;
>> +
>> +       set_pages_state(vaddr, npages, SNP_PAGE_STATE_PRIVATE);
>> +
>> +       pvalidate_pages(vaddr, npages, true);
>> +}
> 
> My testing of this patch shows that a significant amount of time is
> spent using the MSR protocol to change page state, in such a
> significant fashion that it's slower than eagerly accepting all
> memory. The difference gets worse as the RAM size goes up, so I think
> there's some phase problem with the GHCB protocol not getting used
> early enough?

Thank you for testing. Let me see what I can find. I might have to rework 
Brijesh's original patches more to make use of the early boot GHCB in 
order to cut down on the number of MSR protocol requests.

Thanks,
Tom

>