linux-kernel - Re: [PATCH v4 7/8] KVM: VMX: Disable L1TF L1 data cache flush if CONFIG_CPU

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Message-ID: <DDWIH1QNJBN8.2FS1RESKCD3Y@google.com>
Date: Fri, 31 Oct 2025 12:37:24 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 7/8] KVM: VMX: Disable L1TF L1 data cache flush if CONFIG_CPU_MITIGATIONS=n

On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> Disable support for flushing the L1 data cache to mitigate L1TF if CPU
> mitigations are disabled for the entire kernel.  KVM's mitigation of L1TF
> is in no way special enough to justify ignoring CONFIG_CPU_MITIGATIONS=n.
>
> Deliberately use CPU_MITIGATIONS instead of the more precise
> MITIGATION_L1TF, as MITIGATION_L1TF only controls the default behavior,
> i.e. CONFIG_MITIGATION_L1TF=n doesn't completely disable L1TF mitigations
> in the kernel.
>
> Keep the vmentry_l1d_flush module param to avoid breaking existing setups,
> and leverage the .set path to alert the user to the fact that
> vmentry_l1d_flush will be ignored.  Don't bother validating the incoming
> value; if an admin misconfigures vmentry_l1d_flush, the fact that the bad
> configuration won't be detected when running with CONFIG_CPU_MITIGATIONS=n
> is likely the least of their worries.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Brendan Jackman <jackmanb@...gle.com>

(Git says no changed lines)