linux-kernel - Re: [PATCH v2] x86/paravirt: Drop {read,write}

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Date:   Fri, 19 Jul 2019 14:44:07 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Nadav Amit <namit@...are.com>, LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Juergen Gross <jgross@...e.com>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v2] x86/paravirt: Drop {read,write}_cr8() hooks

On 16/07/2019 01:05, Andy Lutomirski wrote:
> On Mon, Jul 15, 2019 at 4:30 PM Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>> On 15/07/2019 19:17, Nadav Amit wrote:
>>>> On Jul 15, 2019, at 8:16 AM, Andrew Cooper <andrew.cooper3@...rix.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