linux-kernel - Re: [PATCHv6 29/30] ACPICA: Avoid cache flush inside virtual machines

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Message-ID: <87ilscd2lw.ffs@tglx>
Date:   Thu, 17 Mar 2022 16:23:55 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv6 29/30] ACPICA: Avoid cache flush inside virtual machines

On Wed, Mar 16 2022 at 05:08, 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>