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Date: Wed, 31 Jan 2024 17:12:05 -0800
From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
	ak@linux.intel.com, tim.c.chen@linux.intel.com,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Nikolay Borisov <nik.borisov@suse.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	kvm@vger.kernel.org,
	Alyssa Milburn <alyssa.milburn@linux.intel.com>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	antonio.gomez.iglesias@linux.intel.com
Subject: Re: [PATCH  v6 6/6] KVM: VMX: Move VERW closer to VMentry for MDS
 mitigation
Message-ID: <20240201011129.rmuhpl6iwpozpll6@desk>
References: <20240123-delay-verw-v6-0-a8206baca7d3@linux.intel.com>
 <20240123-delay-verw-v6-6-a8206baca7d3@linux.intel.com>
 <ZbQkyr8c12jOqWQ-@google.com>
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In-Reply-To: <ZbQkyr8c12jOqWQ-@google.com>

On Fri, Jan 26, 2024 at 01:31:54PM -0800, Sean Christopherson wrote:
> 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@linux.intel.com>
> > ---
> 
> Acked-by: Sean Christopherson <seanjc@google.com>

Thanks.