On Mon, Aug 18, 2025 at 2:07 PM Jan Beulich <jbeul...@suse.com> wrote:
>
> On 15.08.2025 22:41, Andrew Cooper wrote:
> > The TSC functionality is only related to MSRs by write_tsc(), and this
> > really
> > does not want to be available as widely as is currently is.
> >
> > asm/time.h shouldn't be including asm/msr.h, but this turns out to be
> > sufficiently tangled that I've chosen to break it out into it's own patch.
> >
> > No functional change.
> >
> > Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
>
> Acked-by: Jan Beulich <jbeul...@suse.com>
> with one nit:
>
> > --- /dev/null
> > +++ b/xen/arch/x86/include/asm/tsc.h
> > @@ -0,0 +1,46 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +#ifndef X86_TSC_H
> > +#define X86_TSC_H
> > +
> > +#include <asm/alternative.h>
> > +
> > +static inline uint64_t rdtsc(void)
> > +{
> > + uint64_t low, high;
> > +
> > + asm volatile ( "rdtsc" : "=a" (low), "=d" (high) );
> > +
> > + return (high << 32) | low;
> > +}
> > +
> > +static inline uint64_t rdtsc_ordered(void)
> > +{
> > + uint64_t low, high, aux;
> > +
> > + /*
> > + * The RDTSC instruction is not serializing. Make it dispatch
> > serializing
> > + * for the purposes here by issuing LFENCE (or MFENCE if necessary)
> > ahead
> > + * of it.
> > + *
> > + * RDTSCP, otoh, "does wait until all previous instructions have
> > executed
> > + * and all previous loads are globally visible" (SDM) / "forces all
> > older
> > + * instructions to retire before reading the timestamp counter" (APM).
> > + */
> > + alternative_io_2("lfence; rdtsc",
> > + "mfence; rdtsc", X86_FEATURE_MFENCE_RDTSC,
> > + "rdtscp", X86_FEATURE_RDTSCP,
> > + ASM_OUTPUT2("=a" (low), "=d" (high), "=c" (aux)),
> > + /* no inputs */);
> > +
> > + return (high << 32) | low;
> > +}
> > +
> > +#define __write_tsc(val) wrmsrl(MSR_IA32_TSC, val)
> > +#define write_tsc(val) ({ \
> > + /* Reliable TSCs are in lockstep across all CPUs. We should \
> > + * never write to them. */ \
>
> This comment may want to become a proper Xen-style one while being moved.
>
> Jan
>
Does it make sense to move the comment out of the macro?
Frediano
