Re: [PATCH v7 10/10] x86/vm_event: Carry Processor Trace buffer offset in vm_event

[Tamas K Lengyel <tamas@tklengyel.com>]

On Fri, Jan 29, 2021 at 6:22 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>
> On 26/01/2021 14:27, Jan Beulich wrote:
> > On 21.01.2021 22:27, Andrew Cooper wrote:
> >> --- a/xen/arch/x86/vm_event.c
> >> +++ b/xen/arch/x86/vm_event.c
> >> @@ -251,6 +251,9 @@ void vm_event_fill_regs(vm_event_request_t *req)
> >>
> >>      req->data.regs.x86.shadow_gs = ctxt.shadow_gs;
> >>      req->data.regs.x86.dr6 = ctxt.dr6;
> >> +
> >> +    if ( hvm_vmtrace_output_position(curr, &req->data.regs.x86.pt_offset) != 1 )
> >> +        req->data.regs.x86.pt_offset = ~0;
> > Ah. (Regarding my earlier question about this returning -errno or
> > boolean).
> >
> >> --- a/xen/include/public/vm_event.h
> >> +++ b/xen/include/public/vm_event.h
> >> @@ -223,6 +223,12 @@ struct vm_event_regs_x86 {
> >>       */
> >>      uint64_t npt_base;
> >>
> >> +    /*
> >> +     * Current offset in the Processor Trace buffer. For Intel Processor Trace
> >> +     * this is MSR_RTIT_OUTPUT_MASK. Set to ~0 if no Processor Trace is active.
> >> +     */
> >> +    uint64_t pt_offset;
> > According to vmtrace_output_position() the value is only one half
> > of what the named MSR contains. Perhaps "... this is from MSR_..."?
> > Not sure whether, despite this, there still is a reason to have
> > this 64-bit wide.
>
> This is a vestigial remnant which escaped the "use vmtrace uniformly" work.
>
> It should match the domctl_vmtrace_output_position() format, so each
> interface gives the same content for the attempted-platform-neutral version.

From the vm_event ABI perspective it's simpler to have a 64-bit value
here even if the max value it may possibly carry is never going to use
the whole 64-bit width. I rather not play with shortening it just to
add padding somewhere else.

As for what it's called is not that important from my perspective,
vmtrace_pos or something like that for example is fine by me.

Tamas