Re: [PATCH v2] x86/paravirt: Drop {read,write}_cr8() hooks

[Andy Lutomirski]

On Mon, Jul 15, 2019 at 4:30 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 15/07/2019 19:17, Nadav Amit wrote:
> >> On Jul 15, 2019, at 8:16 AM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 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 APICâ):
> >
> > "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.â
> >
> > 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.