linux-kernel - Re: [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

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Message-ID: <8ab309cd-8465-d543-55c8-5f6529fe74fd@intel.com>
Date:   Mon, 12 Jul 2021 12:00:01 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Gonda <pgonda@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        kvm list <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>,
        Sean Christopherson <seanjc@...gle.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 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,
        Nathaniel McCallum <npmccallum@...hat.com>,
        brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for
 RMPUPDATE and PSMASH instruction

On 7/12/21 11:44 AM, Peter Gonda wrote:
>> +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.

It's probably fine to just leave this.  While you could *theoretically*
lose this race forever, it's unlikely to happen in practice.  If it
does, you'll get an easy-to-understand softlockup backtrace which should
point here pretty quickly.

I think TDX has a few of these as well.  Most of the "SEAMCALL"s from
host to the firmware doing the security enforcement have something like
an -EBUSY as well.  I believe they just retry forever too.