Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Received-SPF: None (esa3.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  andrew.cooper3@citrix.com) identity=pra;
  client-ip=162.221.158.21; receiver=esa3.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="andrew.cooper3@citrix.com";
  x-conformance=sidf_compatible
Received-SPF: Pass (esa3.hc3370-68.iphmx.com: domain of
  Andrew.Cooper3@citrix.com designates 162.221.158.21 as
  permitted sender) identity=mailfrom;
  client-ip=162.221.158.21; receiver=esa3.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="Andrew.Cooper3@citrix.com";
  x-conformance=sidf_compatible; x-record-type="v=spf1";
  x-record-text="v=spf1 ip4:209.167.231.154 ip4:178.63.86.133
  ip4:195.66.111.40/30 ip4:85.115.9.32/28 ip4:199.102.83.4
  ip4:192.28.146.160 ip4:192.28.146.107 ip4:216.52.6.88
  ip4:216.52.6.188 ip4:162.221.158.21 ip4:162.221.156.83 ~all"
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  authenticity information available from domain of
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  client-ip=162.221.158.21; receiver=esa3.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="postmaster@mail.citrix.com";
  x-conformance=sidf_compatible
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 Brs=
Subject: Re: [PATCH v2] x86/paravirt: Drop {read,write}_cr8() hooks
To:     Andy Lutomirski <luto@kernel.org>
CC:     Nadav Amit <namit@vmware.com>, LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephane Eranian <eranian@google.com>,
        Feng Tang <feng.tang@intel.com>,
        Juergen Gross <jgross@suse.com>,
        "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Pavel Machek <pavel@ucw.cz>
References: <20190715151641.29210-1-andrew.cooper3@citrix.com>
 <602B4D96-E2A9-45BE-8247-4E3481ED5402@vmware.com>
 <4a7592c8-0ee8-f394-c445-4032daf74493@citrix.com>
 <CALCETrWBO6dUNzkyD12ZfEQ+biN8rhrWxm8YoNhgisDd2Spujg@mail.gmail.com>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
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Message-ID: <1b6ef9c2-6c55-81b9-2b9d-4dc7b5d764ab@citrix.com>
Date:   Fri, 19 Jul 2019 14:44:07 +0100
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On 16/07/2019 01:05, Andy Lutomirski wrote:
> On Mon, Jul 15, 2019 at 4:30 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> On 15/07/2019 19:17, Nadav Amit wrote:
>>>> On Jul 15, 2019, at 8:16 AM, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>>>
>>>> There is a lot of infrastructure for functionality which is used
>>>> exclusively in __{save,restore}_processor_state() on the suspend/resume
>>>> path.
>>>>
>>>> cr8 is an alias of APIC_TASKPRI, and APIC_TASKPRI is saved/restored by
>>>> lapic_{suspend,resume}().  Saving and restoring cr8 independently of the
>>>> rest of the Local APIC state isn't a clever thing to be doing.
>>>>
>>>> Delete the suspend/resume cr8 handling, which shrinks the size of struct
>>>> saved_context, and allows for the removal of both PVOPS.
>>> I think removing the interface for CR8 writes is also good to avoid
>>> potential correctness issues, as the SDM says (10.8.6.1 "Interaction of Task
>>> Priorities between CR8 and APIC”):
>>>
>>> "Operating software should implement either direct APIC TPR updates or CR8
>>> style TPR updates but not mix them. Software can use a serializing
>>> instruction (for example, CPUID) to serialize updates between MOV CR8 and
>>> stores to the APIC.”
>>>
>>> And native_write_cr8() did not even issue a serializing instruction.
>>>
>> Given its location, the one write_cr8() is bounded by two serialising
>> operations, so is safe in practice.
>>
>> However, I agree with the statement in the manual.  I could submit a v3
>> with an updated commit message, or let it be fixed on commit.  Whichever
>> is easiest.
>>
> I don't see anything wrong with the message.  If we actually used CR8
> for interrupt priorities, we wouldn't want it to serialize.  The bug
> is that the code that did the write_cr8() should have had a comment as
> to how it serialized against lapic_restore().  But that doesn't seem
> worth mentioning in the message, since, as noted, the real problem was
> that it nonsensically restored just TPR without restoring everything
> else.

Fair enough, in which case I'm happy with v2 as it is.

~Andrew