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Re: [Xen-devel] [PATCH 1/4] x86: Begin to remove TSC mode PVRDTSCP
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 20 Nov 2018 11:14:34 +0000
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- Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 20 Nov 2018 11:14:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 19/11/2018 15:25, Jan Beulich wrote:
>>>> On 15.11.18 at 22:47, <andrew.cooper3@xxxxxxxxxx> wrote:
>> For more historical context, see
>> c/s c17b36d5dc792cfdf59b6de0213b168bec0af8e8
>> c/s 04656384a1b9714e43db850c51431008e23450d8
>>
>> PVRDTSCP was an attempt to provide Xen-aware userspace with a stable
>> monotonic
>> clock, and enough information for said userspace to cope with frequency
>> changes across migrate. However, the PVRDTSCP infrastructure has resulted in
>> very tangled code, and non-architectural behaviour even in non-PVRDTSCP
>> cases.
>>
>> Seeing as the functionality has been replaced entirely by improvements in PV
>> clocks (including being plumbed into the VDSO nowadays), or alternatively by
>> hardware TSC scaling features, and no-one is aware of any remaining users of
>> this mode, take the opportunity to remove it.
> With "remaining" suitably replaced to express that none are known to
> ever have existed in the wild ...
>
>> For now, introduce an upper range check on the toolstack setting to
>> XEN_DOMCTL_settscinfo, and exclude TSC_MODE_PVRDTSCP from selection.
>> (Arguably, its a bug that this hypercall previously accepted any value and
>> turned into a nop). This will catch and cleanly reject attempts to migrate
>> in
>> a VM configured to use PVRDTSCP, rather than letting it run and have the
>> wrong
>> timing mode.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit I think this ...
>
>> --- a/xen/arch/x86/domctl.c
>> +++ b/xen/arch/x86/domctl.c
>> @@ -970,7 +970,8 @@ long arch_do_domctl(
>> break;
>>
>> case XEN_DOMCTL_settscinfo:
>> - if ( d == currd ) /* no domain_pause() */
>> + if ( d == currd || /* no domain_pause() */
>> + domctl->u.tsc_info.tsc_mode > TSC_MODE_NEVER_EMULATE )
> ... would better be left to tsc_set_info().
I think I'll do a separate prerequisite patch which causes
tsc_set_info() not to be void.
~Andrew
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