linux-kernel - Re: [PATCH v6 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

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Date: Fri, 26 Jan 2024 13:31:54 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Andy Lutomirski <luto@...nel.org>, Jonathan Corbet <corbet@....net>, 
	Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com, ak@...ux.intel.com, 
	tim.c.chen@...ux.intel.com, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, kvm@...r.kernel.org, 
	Alyssa Milburn <alyssa.milburn@...ux.intel.com>, 
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH  v6 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

On Tue, Jan 23, 2024, Pawan Gupta wrote:
> During VMentry VERW is executed to mitigate MDS. After VERW, any memory
> access like register push onto stack may put host data in MDS affected
> CPU buffers. A guest can then use MDS to sample host data.
> 
> Although likelihood of secrets surviving in registers at current VERW
> callsite is less, but it can't be ruled out. Harden the MDS mitigation
> by moving the VERW mitigation late in VMentry path.
> 
> Note that VERW for MMIO Stale Data mitigation is unchanged because of
> the complexity of per-guest conditional VERW which is not easy to handle
> that late in asm with no GPRs available. If the CPU is also affected by
> MDS, VERW is unconditionally executed late in asm regardless of guest
> having MMIO access.
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---

Acked-by: Sean Christopherson <seanjc@...gle.com>