Re: [PATCH 5.15.y] KVM: arm64: vgic-v4: Make the doorbell request robust w.r.t preemption

[Marc Zyngier <maz@kernel.org>]

On Mon, 01 Jul 2024 12:19:33 +0100,
James Gowans <jgowans@amazon.com> wrote:
> 
> From: Marc Zyngier <maz@kernel.org>
> 
> Xiang reports that VMs occasionally fail to boot on GICv4.1 systems when
> running a preemptible kernel, as it is possible that a vCPU is blocked
> without requesting a doorbell interrupt.
> 
> The issue is that any preemption that occurs between vgic_v4_put() and
> schedule() on the block path will mark the vPE as nonresident and *not*
> request a doorbell irq. This occurs because when the vcpu thread is
> resumed on its way to block, vcpu_load() will make the vPE resident
> again. Once the vcpu actually blocks, we don't request a doorbell
> anymore, and the vcpu won't be woken up on interrupt delivery.
> 
> Fix it by tracking that we're entering WFI, and key the doorbell
> request on that flag. This allows us not to make the vPE resident
> when going through a preempt/schedule cycle, meaning we don't lose
> any state.
> 
> Cc: stable@vger.kernel.org
> Fixes: 8e01d9a396e6 ("KVM: arm64: vgic-v4: Move the GICv4 residency flow to be driven by vcpu_load/put")
> Reported-by: Xiang Chen <chenxiang66@hisilicon.com>
> Suggested-by: Zenghui Yu <yuzenghui@huawei.com>
> Tested-by: Xiang Chen <chenxiang66@hisilicon.com>
> Co-developed-by: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Acked-by: Zenghui Yu <yuzenghui@huawei.com>
> Link: https://lore.kernel.org/r/20230713070657.3873244-1-maz@kernel.org
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> 
> (cherry picked from commit b321c31c9b7b309dcde5e8854b741c8e6a9a05f0)
> 
> [modified to wrangle the vCPU flags directly instead of going through
> the flag helper macros as they have not yet been introduced. Also doing
> the flag wranging in the kvm_arch_vcpu_{un}blocking() hooks as the
> introduction of kvm_vcpu_wfi has not yet happened. See:
> 6109c5a6ab7f ("KVM: arm64: Move vGIC v4 handling for WFI out arch callback hook")]
> 
> Signed-off-by: James Gowans <jgowans@amazon.com>

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
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