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Date:   Wed, 16 Mar 2022 15:13:18 -0700
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Subject: Re: [PATCHv6 29/30] ACPICA: Avoid cache flush inside virtual machines
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        luto@kernel.org, peterz@infradead.org
Cc:     sathyanarayanan.kuppuswamy@linux.intel.com, aarcange@redhat.com,
        ak@linux.intel.com, dan.j.williams@intel.com, david@redhat.com,
        hpa@zytor.com, jgross@suse.com, jmattson@google.com,
        joro@8bytes.org, jpoimboe@redhat.com, knsathya@kernel.org,
        pbonzini@redhat.com, sdeep@vmware.com, seanjc@google.com,
        tony.luck@intel.com, vkuznets@redhat.com, wanpengli@tencent.com,
        thomas.lendacky@amd.com, brijesh.singh@amd.com, x86@kernel.org,
        linux-kernel@vger.kernel.org
References: <20220316020856.24435-1-kirill.shutemov@linux.intel.com>
 <20220316020856.24435-30-kirill.shutemov@linux.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20220316020856.24435-30-kirill.shutemov@linux.intel.com>
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Precedence: bulk

On 3/15/22 19:08, Kirill A. Shutemov wrote:
> While running inside virtual machine, the kernel can bypass cache
> flushing. Changing sleep state in a virtual machine doesn't affect the
> host system sleep state and cannot lead to data loss.
> 
> Before entering sleep states, the ACPI code flushes caches to prevent
> data loss using the WBINVD instruction.  This mechanism is required on
> bare metal.
> 
> But, any use WBINVD inside of a guest is worthless.  Changing sleep
> state in a virtual machine doesn't affect the host system sleep state
> and cannot lead to data loss, so most hypervisors simply ignore it.
> Despite this, the ACPI code calls WBINVD unconditionally anyway.
> It's useless, but also normally harmless.
> 
> In TDX guests, though, WBINVD stops being harmless; it triggers a
> virtualization exception (#VE).  If the ACPI cache-flushing WBINVD
> were left in place, TDX guests would need handling to recover from
> the exception.
> 
> Avoid using WBINVD whenever running under a hypervisor.  This both
> removes the useless WBINVDs and saves TDX from implementing WBINVD
> handling.

Looks good.  Did you have more acks on this earlier that got removed?  I
thought I remembered more acks on earlier versions.

Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>