Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220614120231.48165-1-kirill.shutemov@linux.intel.com>
 <cover.1659103274.git.thomas.lendacky@amd.com> <44495aa8acb666b447a08a1c3af80987aa3cea3a.1659103274.git.thomas.lendacky@amd.com>
In-Reply-To: <44495aa8acb666b447a08a1c3af80987aa3cea3a.1659103274.git.thomas.lendacky@amd.com>
From:   Dionna Amalie Glaze <dionnaglaze@google.com>
Date:   Mon, 22 Aug 2022 17:24:24 -0700
Message-ID: <CAAH4kHakyK06eqQhaagdMmvPq6vsWK51k-f4UgUZWzexZOuBLg@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] x86/sev: Add SNP-specific unaccepted memory support
To:     Tom Lendacky <thomas.lendacky@amd.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Michael Roth <michael.roth@amd.com>,
        Joerg Roedel <jroedel@suse.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

>
> +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)