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Re: [PATCH 4/5] x86/HVM: drop tsc_scaling.setup() hook
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 22 Nov 2023 10:13:08 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>
- Delivery-date: Wed, 22 Nov 2023 09:13:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.11.2023 10:06, Roger Pau Monné wrote:
> On Thu, Nov 16, 2023 at 02:32:47PM +0100, Jan Beulich wrote:
>> This was used by VMX only, and the intended VMCS write can as well
>> happen from vmx_set_tsc_offset(), invoked (directly or indirectly)
>> almost immediately after the present call sites of the hook.
>> vmx_set_tsc_offset() isn't invoked frequently elsewhere, so the extra
>> VMCS write shouldn't raise performance concerns.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks.
> I would be lying if I told you I understand guest TSC management in
> Xen, but patch does keep the previous call sites with adding others
> that are indeed not hot paths.
>
> I do wonder whether it would be possible to set this before the domain
> is started, as AFAICT the TSC scaling ratio is only set before the
> domain is started (either by domain create or other toolstack
> hypercalls).
>
> Could we maybe further limit the write to the case where the vCPU is
> not running?
Possibly, but not straightforwardly: E.g. I think we'd need to actually
enforce tsc_set_info() to only ever act on non-running (perhaps even
not-yet-started) domains. And I mean not by looking where it's being
called from at present (which may already provide such kind-of-
guarantees), but in the function itself.
Jan
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