Re: [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Keir Fraser [mailto:keir.xen@gmail.com]
> Subject: Re: [Xen-devel] [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by
> hardware
> 
> On 20/04/2012 03:21, "Boris Ostrovsky" <boris.ostrovsky@amd.com> wrote:
> 
> > # HG changeset patch
> > # User Boris Ostrovsky <boris.ostrovsky@amd.com>
> > # Date 1334875170 14400
> > # Node ID 55bf11ebce87ceb73fb2c372dcef170ec0bb4a18
> > # Parent  7c777cb8f705411b77c551f34ba88bdc09e38ab8
> > svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware
> >
> > When running in TSC_MODE_ALWAYS_EMULATE mode on processors that support
> > TSC scaling we don't need to intercept RDTSC/RDTSCP instructions.
> >
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
> > Acked-by: Wei Huang <wei.huang2@amd.com>
> > Tested-by: Wei Huang <wei.huang2@amd.com>
> 
> Worth an ack/nack from Dan M I'd say. He'll probably have some comment about
> possible cross-CPU TSC skew.

Provided the hypervisor writes the "TSC-scale-register" with the same
value when any vcpu for any guest is scheduled, I don't think
there is any cross-CPU TSC skew.

But my recollection is that I had a concern that, to work properly
after migration, TSC scaling might need to implement:

	((B + cur_tsc) * scale ) + A

and AFAIK the feature only implements this for B==0.
Without the rest of the implementation in the hypervisor
and tools, it's hard to determine whether my concern is valid
or not.

Also, I don't recall the exact scaling process but
was also concerned that a guest kernel or userland
process approximating the passage of time by counting
TSC cycles, they might just estimate the value once at
boot (or application startup) and, due to the scaling,
post-migration ticks/second might later change enough
to be a problem.  However, this
is a generic problem with emulation as well, so the
concern is really: Does the TSC scaling use the same
multiplication precision as is available to emulation
in the hypervisor?

Dan