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From:   Peter Gonda <pgonda@google.com>
Date:   Mon, 12 Jul 2021 12:44:24 -0600
Message-ID: <CAMkAt6quzRMiEJ=iYDocRvpaYuNcV5vm=swbowK+KG=j7FjyKA@mail.gmail.com>
Subject: Re: [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for
 RMPUPDATE and PSMASH instruction
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        kvm list <kvm@vger.kernel.org>, linux-efi@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev,
        linux-mm@kvack.org, linux-crypto@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>, tony.luck@intel.com,
        Nathaniel McCallum <npmccallum@redhat.com>,
        brijesh.ksingh@gmail.com
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> +int psmash(struct page *page)
> +{
> +       unsigned long spa = page_to_pfn(page) << PAGE_SHIFT;
> +       int ret;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +               return -ENXIO;
> +
> +       /* Retry if another processor is modifying the RMP entry. */
> +       do {
> +               /* Binutils version 2.36 supports the PSMASH mnemonic. */
> +               asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> +                             : "=a"(ret)
> +                             : "a"(spa)
> +                             : "memory", "cc");
> +       } while (ret == FAIL_INUSE);

Should there be some retry limit here for safety? Or do we know that
we'll never be stuck in this loop? Ditto for the loop in rmpupdate.

> +
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(psmash);
>