Bug 2016966

Summary:	amd/x86: Advertise NullSelectClearsBase on Rome and Milan
Product:	Red Hat Enterprise Linux 9	Reporter:	Dr. David Alan Gilbert <dgilbert>
Component:	qemu-kvm	Assignee:	Virtualization Maintenance <virt-maint>
qemu-kvm sub component:	CPU Models	QA Contact:	Li Xiaohui <xiaohli>
Status:	CLOSED MIGRATED	Docs Contact:
Severity:	medium
Priority:	medium	CC:	babu.moger, bdas, coli, ctatman, jinzhao, juzhang, nanliu, nilal, quintela, virt-maint, wehuang, ymankad
Version:	9.0	Keywords:	MigratedToJIRA, Reopened, Triaged
Target Milestone:	rc	Flags:	pm-rhel: mirror+
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2023-09-22 13:40:08 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Dr. David Alan Gilbert 2021-10-25 10:04:52 UTC

Description of problem:
There's a new CPUId bit 80000021/bit 6
'NullSelectClearsBase - Null segment selector loads also clear the destination segment register base and limit'
guests currently detect this behaviour at run time, but  there's an upstream fix that starts using this bit and bz 2016959/2016961 request backports; once we start using this bit we should advertise if the cpu has it.
Milan apparently does.
Rome doesn't advertise the bit but is believed to have actually fixed it but not advertise it.

We should advertise the bit in the Milan and Rome models.


Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1.
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Actual results:


Expected results:


Additional info:

Comment 1 Dr. David Alan Gilbert 2021-10-25 10:07:22 UTC

Wei:
  Can you point to something that confirms Rome's behaviour on this?

Comment 2 John Ferlan 2021-11-03 19:27:07 UTC

Assigned to Amnon for initial triage per bz process and age of bug created or assigned to virt-maint without triage.

Comment 3 WEI HUANG 2021-11-15 19:16:55 UTC

Babu is looking into this. Could you open access to BZ2016959/BZ2016961 to AMD?

Comment 4 Dr. David Alan Gilbert 2021-11-16 10:11:16 UTC

(In reply to WEI HUANG from comment #3)
> Babu is looking into this. Could you open access to BZ2016959/BZ2016961 to
> AMD?

Done.

Comment 5 Babu Moger 2022-01-10 14:57:50 UTC

Hi David, 
The CPUID feature NullSelectClearsBase (Function 80000021/bit 6) is available on Milan and future products. I will plan to add this bit in EPYC-Milan.

However, this feature is not available in Rome platform. So, we cannot add it in EPYC-Rome.

Thanks
Babu

Comment 10 liunana 2023-04-24 10:11:58 UTC

Hi Bandan,

Is this one RFE bug according to Commen 0 and Comment 5?
If so pleae help add the FutureFeature keywords. Thanks.



Best regards
Nana

Comment 11 Li Xiaohui 2023-04-24 10:28:15 UTC

Hi Nitesh, Bandan, 
According to this bug's Description,
'NullSelectClearsBase - Null segment selector loads also clear the destination segment register base and limit'

When migrating VM (specify the cpu to be EPYC) from Rome/Milan to Naples, do you know what migration issue would happen?

Comment 12 Bandan Das 2023-04-24 14:49:14 UTC

(In reply to Li Xiaohui from comment #11)
> Hi Nitesh, Bandan, 
> According to this bug's Description,
> 'NullSelectClearsBase - Null segment selector loads also clear the
> destination segment register base and limit'
> 
> When migrating VM (specify the cpu to be EPYC) from Rome/Milan to Naples, do
> you know what migration issue would happen?

First, to respond to Nana, it looks more like a bug fix to me.

Regarding migration, I am not entirely sure but from the kernel patch, the assumption is 
that guest wouldn't be able to detect the virtual cpuid bit when migrating to Naples and 
so would be aware of the X86_BUG_NULL_SEG bug. Maybe Juan is aware of 
this ?

Comment 13 RHEL Program Management 2023-04-25 07:28:19 UTC

After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.

Comment 15 Juan Quintela 2023-05-09 17:20:48 UTC

Hi
I redirect this one to Leonardo, he is the one that have looked at CPUid's and AMD machines in recent times.

Leonardo?

Comment 16 Babu Moger 2023-05-09 17:30:02 UTC

This series addresses this problem for Milan and Genoa processors.
https://lore.kernel.org/kvm/20230504205313.225073-1-babu.moger@amd.com/#r
Paolo is planning to pull this series for the next release.

Comment 20 RHEL Program Management 2023-09-22 13:39:05 UTC

Issue migration from Bugzilla to Jira is in process at this time. This will be the last message in Jira copied from the Bugzilla bug.

Comment 21 RHEL Program Management 2023-09-22 13:40:08 UTC

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