Re: [Xen-devel] [PATCH v9] new config option vtsc_tolerance_khz to avoid TSC emulation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.06.18 at 15:47, <olaf@xxxxxxxxx> wrote:
> Am Thu, 07 Jun 2018 07:31:26 -0600
> schrieb "Jan Beulich" <JBeulich@xxxxxxxx>:
> 
>> Hmm, I find this description concerning. Is the field reliably zero when
>> coming from older Xen, or is it not?
> 
> It depends on how old.
> If the other sending side has write_tsc_info from 4.6+, then _res1 is zero.
> If it is older, the XC_SAVE_ID_TSC_INFO is eventually not handled anymore.
> At least I could not find the relevant code, even in staging-4.6.
> But then, it is unlikely that migration from 4.5 to 4.6 did not work.
> 
> Also, in general the input comes from "remote". So it is untrusted.

But isn't this exactly the concern Andrew had voiced? If so, we're not
any closer to this having a chance to go in. The re-use of the field is
acceptable only if all existing senders reliably fill zero in there.

Jan



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