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Message-ID: <121a948a7089c7b30226efd6d13871432eec43de.camel@redhat.com>
Date: Thu, 08 Dec 2022 23:56:42 +0200
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v4 15/32] Revert "KVM: SVM: Use target APIC ID to
complete x2AVIC IRQs when possible"
On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Due to a likely mismerge of patches, KVM ended up with a superfluous
> commit to "enable" AVIC's fast path for x2AVIC mode. Even worse, the
> superfluous commit has several bugs and creates a nasty local shadow
> variable.
>
> Rather than fix the bugs piece-by-piece[*] to achieve the same end
> result, revert the patch wholesale.
>
> Opportunistically add a comment documenting the x2AVIC dependencies.
>
> This reverts commit 8c9e639da435874fb845c4d296ce55664071ea7a.
>
> [*] https://lore.kernel.org/all/YxEP7ZBRIuFWhnYJ@google.com
>
> Fixes: 8c9e639da435 ("KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible")
> Suggested-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/kvm/svm/avic.c | 29 +++++++++++------------------
> 1 file changed, 11 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index e35e9363e7ff..605c36569ddf 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -378,7 +378,17 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>
> logid_index = cluster + __ffs(bitmap);
>
> - if (!apic_x2apic_mode(source)) {
> + if (apic_x2apic_mode(source)) {
> + /*
> + * For x2APIC, the logical APIC ID is a read-only value
> + * that is derived from the x2APIC ID, thus the x2APIC
> + * ID can be found by reversing the calculation (done
> + * above). Note, bits 31:20 of the x2APIC ID are not
> + * propagated to the logical ID, but KVM limits the
> + * x2APIC ID limited to KVM_MAX_VCPU_IDS.
> + */
> + l1_physical_id = logid_index;
> + } else {
> u32 *avic_logical_id_table =
> page_address(kvm_svm->avic_logical_id_table_page);
>
> @@ -393,23 +403,6 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>
> l1_physical_id = logid_entry &
> AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK;
> - } else {
> - /*
> - * For x2APIC logical mode, cannot leverage the index.
> - * Instead, calculate physical ID from logical ID in ICRH.
> - */
> - int cluster = (icrh & 0xffff0000) >> 16;
> - int apic = ffs(icrh & 0xffff) - 1;
> -
> - /*
> - * If the x2APIC logical ID sub-field (i.e. icrh[15:0])
> - * contains anything but a single bit, we cannot use the
> - * fast path, because it is limited to a single vCPU.
> - */
> - if (apic < 0 || icrh != (1 << apic))
> - return -EINVAL;
> -
> - l1_physical_id = (cluster << 4) + apic;
> }
> }
>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Best regards,
Maxim Levitsky
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