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From: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
To: Maxim Levitsky <mlevitsk@redhat.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: pbonzini@redhat.com, seanjc@google.com, joro@8bytes.org,
	jon.grimm@amd.com, wei.huang2@amd.com, terry.bowman@amd.com
Subject: Re: [PATCH v2 11/12] KVM: SVM: Do not inhibit APICv when x2APIC is present
Date: Tue, 26 Apr 2022 09:25:03 +0700	[thread overview]
Message-ID: <01460b72-1189-fef1-9718-816f2f658d42@amd.com> (raw)
In-Reply-To: <3fd0aabb6288a5703760da854fd6b09a485a2d69.camel@redhat.com>

Hi Maim,

On 4/19/22 8:29 PM, Maxim Levitsky wrote:
> On Tue, 2022-04-12 at 06:58 -0500, Suravee Suthikulpanit wrote:
> 
> Hi!
> 
> 
> I just got an idea, while writing a kvm selftest that would use AVIC,
> and finding out that selftest code uploads the '-host' cpuid right away
> which has x2apic enabled and that inhibits AVIC, and later clearing x2apic
> in the cpuid doesn't un-inhibit it.
>   
> That can be fixed in few ways but that got me thinking:
>   
> Why do we inhibit AVIC when the guest uses x2apic, even without X2AVIC?
> I think that if we didn't it would just work, and even work faster than
> pure software x2apic.
>   
> My thinking is:
>   
> - when a vcpu itself uses its x2apic, even if its avic is not inhibited,
> the guest will write x2apic msrs which kvm intercepts and will correctly emulate a proper x2apic.
>   
> - vcpu peers will also use x2apic msrs and again it will work correctly
> (even when there are more than 256 vcpus).
>   
> - and the host + iommu will still be able to use AVIC's doorbell to send interrupts to the guest
> and that doesn't need apic ids or anything, it should work just fine.
> 
> Also AVIC should have no issues scanning IRR and injecting interrupts on VM entry,
> x2apic mode doesn't matter for that.
>   
> AVIC mmio can still be though discovered by the guest which is technically against x86 spec
> (in x2apic mode, mmio supposed to not work) but that can be fixed easily by disabing
> the AVIC memslot if any of the vCPUs are in x2apic mode, or this can be ignored since
> it should not cause any issues.
> We seem to have a quirk for that KVM_X86_QUIRK_LAPIC_MMIO_HOLE.
>   
> On top of all this, removing this inhibit will also allow to test AVIC with guest
> which does have x2apic in the CPUID but doesn't use it (e.g kvm unit test, or
> linux booted with nox2apic, which is also nice IMHO)
>   
> What do you think?

This is actually a good idea!!! Let's call it hybrid-x2AVIC :)

I am working on prototype and test out the support for this, which will be introduced in V3.

Regards,
Suravee

  reply	other threads:[~2022-04-26  2:25 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-12 11:58 [PATCH v2 00/12] Introducing AMD x2APIC Virtualization (x2AVIC) support Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 01/12] x86/cpufeatures: Introduce x2AVIC CPUID bit Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 02/12] KVM: x86: lapic: Rename [GET/SET]_APIC_DEST_FIELD to [GET/SET]_XAPIC_DEST_FIELD Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 03/12] KVM: SVM: Detect X2APIC virtualization (x2AVIC) support Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 04/12] KVM: SVM: Update max number of vCPUs supported for x2AVIC mode Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 05/12] KVM: SVM: Update avic_kick_target_vcpus to support 32-bit APIC ID Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 06/12] KVM: SVM: Do not support updating APIC ID when in x2APIC mode Suravee Suthikulpanit
2022-04-18 11:09   ` Maxim Levitsky
2022-04-12 11:58 ` [PATCH v2 07/12] KVM: SVM: Adding support for configuring x2APIC MSRs interception Suravee Suthikulpanit
2022-04-18 11:17   ` Maxim Levitsky
2022-04-12 11:58 ` [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing APIC mode Suravee Suthikulpanit
2022-04-18 12:55   ` Maxim Levitsky
2022-05-02 14:07     ` Suravee Suthikulpanit
2022-05-02 17:13       ` Maxim Levitsky
2022-05-03 13:04         ` Suravee Suthikulpanit
2022-05-04 11:46           ` Maxim Levitsky
2022-05-04 11:49             ` Maxim Levitsky
2022-05-04 12:38               ` Maxim Levitsky
2022-04-12 11:58 ` [PATCH v2 09/12] KVM: SVM: Introduce helper functions to (de)activate AVIC and x2AVIC Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 10/12] KVM: SVM: Do not throw warning when calling avic_vcpu_load on a running vcpu Suravee Suthikulpanit
2022-04-12 11:58 ` [PATCH v2 11/12] KVM: SVM: Do not inhibit APICv when x2APIC is present Suravee Suthikulpanit
2022-04-19 13:29   ` Maxim Levitsky
2022-04-26  2:25     ` Suravee Suthikulpanit [this message]
2022-04-26  7:06       ` Maxim Levitsky
2022-04-26  9:56         ` Maxim Levitsky
2022-04-29 17:00           ` Sean Christopherson
2022-05-01  6:49             ` Maxim Levitsky
2022-04-12 11:58 ` [PATCH v2 12/12] kvm/x86: Remove APICV activate mode inconsistency check Suravee Suthikulpanit
2022-04-18 12:55   ` Maxim Levitsky

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