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

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Message-ID: <CAAH4kHakyK06eqQhaagdMmvPq6vsWK51k-f4UgUZWzexZOuBLg@mail.gmail.com>
Date:   Mon, 22 Aug 2022 17:24:24 -0700
From:   Dionna Amalie Glaze <dionnaglaze@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....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

>
> +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?

-- 
-Dionna Glaze, PhD (she/her)