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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 14:50:18 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
- Delivery-date: Tue, 14 May 2024 13:50:38 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14/05/2024 12:09 pm, Andrew Cooper wrote:
> 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>
Actually no - this isn't safe in all cases.
There are BIOSes which get MTRRs wrong, and with the APs having UC
covering a wider region than the BSP.
In this case, creating consistency will alter the MTRRs on all CPUs
currently up, and we do need to perform the rendezvous in that case.
There are 3 cases:
1) Nothing to do. This is the overwhemlingly common case.
2) Local changes only. No broadcast, but we do need to enter CD mode.
3) Remote changes needed. Needs full broadcast.
~Andrew
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