linux-kernel - Re: [PATCH] KVM: x86: Document a virtualization gap for GIF on AMD CPUs

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Message-ID: <aQP-Qv1SOi0cy-nO@archie.me>
Date: Fri, 31 Oct 2025 07:09:38 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Document a virtualization gap for GIF on AMD
 CPUs

On Thu, Oct 30, 2025 at 10:37:57PM +0000, Yosry Ahmed wrote:
> -TBD
> +On AMD CPUs, when GIF is cleared, #DB exceptions or traps due to a breakpoint
> +register match are ignored and discarded by the CPU. The CPU relies on the VMM
> +to fully virtualize this behavior, even when vGIF is enabled for the guest
> +(i.e. vGIF=0 does not cause the CPU to drop #DBs when the guest is running).
> +KVM does not virtualize this behavior as the complexity is unjustified given
> +the rarity of the use case. One way to handle this would be for KVM to
> +intercept the #DB, temporarily disable the breakpoint, single-step over the
> +instruction, then re-enable the breakpoint.

The wording LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>

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