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

From: Nadav Amit
Date: Mon Jul 15 2019 - 14:18:02 EST

> 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 ( "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.