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Re: [PATCH for-4.19] x86/mtrr: avoid system wide rendezvous when setting AP MTRRs


  • To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 14 May 2024 12:09:35 +0100
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  • Cc: Jan Beulich <jbeulich@xxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
  • Delivery-date: Tue, 14 May 2024 11:09:58 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 13/05/2024 9:59 am, Roger Pau Monne wrote:
> There's no point in forcing a system wide update of the MTRRs on all 
> processors
> when there are no changes to be propagated.  On AP startup it's only the AP
> that needs to write the system wide MTRR values in order to match the rest of
> the already online CPUs.
>
> We have occasionally seen the watchdog trigger during `xen-hptool cpu-online`
> in one Intel Cascade Lake box with 448 CPUs due to the re-setting of the MTRRs
> on all the CPUs in the system.
>
> While there adjust the comment to clarify why the system-wide resetting of the
> MTRR registers is not needed for the purposes of mtrr_ap_init().
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> For consideration for 4.19: it's a bugfix of a rare instance of the watchdog
> triggering, but it's also a good performance improvement when performing
> cpu-online.
>
> Hopefully runtime changes to MTRR will affect a single MSR at a time, lowering
> the chance of the watchdog triggering due to the system-wide resetting of the
> range.

"Runtime" changes will only be during dom0 boot, if at all, but yes - it
is restricted to a single MTRR at a time.

It's XENPF_{add,del,read}_memtype, but it's only used by Classic Linux. 
PVOps only issues read_memtype.

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>



 


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