Re: [PATCH 06/10] KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration

[Paul Durrant]

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> The documentation on TSC migration using KVM_VCPU_TSC_OFFSET is woefully
> inadequate. It ignores TSC scaling, and ignores the fact that the host
> TSC may differ from one host to the next (and in fact because of the way
> the kernel calibrates it, it generally differs from one boot to the next
> even on the same hardware).
>
> Add KVM_VCPU_TSC_SCALE to extract the actual scale ratio and frac_bits,
> and attempt to document the *awful* process that we're requiring userspace
> to follow to merely preserve the TSC across migration.
>
> I may have thrown up in my mouth a little when writing that documentation.
> It's an awful API. If we do this, we should be ashamed of ourselves.
> (I also haven't tested the documented process yet).
>
> Let's use Simon's KVM_VCPU_TSC_VALUE instead.
> https://lore.kernel.org/all/20230202165950.483430-1-sveith@xxxxxxxxx/
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   Documentation/virt/kvm/devices/vcpu.rst | 115 ++++++++++++++++++------
>   arch/x86/include/uapi/asm/kvm.h         |   6 ++
>   arch/x86/kvm/x86.c                      |  15 ++++
>   3 files changed, 109 insertions(+), 27 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>