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

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Date:   Tue, 16 Jul 2019 00:30:09 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Nadav Amit <namit@...are.com>
CC:     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>,
        Andy Lutomirski <luto@...nel.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 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.

~Andrew