linux-kernel - Re: [PATCHv4 29/30] ACPICA: Avoid cache flush on TDX guest

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Message-ID: <20220228170224.igzvf7e3tc6ujrfq@treble>
Date:   Mon, 28 Feb 2022 09:02:24 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, sdeep@...are.com,
        Sean Christopherson <seanjc@...gle.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv4 29/30] ACPICA: Avoid cache flush on TDX guest

On Mon, Feb 28, 2022 at 07:37:13PM +0300, Kirill A. Shutemov wrote:
> On Sun, Feb 27, 2022 at 05:34:45PM -0800, Dan Williams wrote:
> > On Sun, Feb 27, 2022 at 2:05 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > On Thu, Feb 24, 2022 at 06:56:29PM +0300, Kirill A. Shutemov wrote:
> > > > +/*
> > > > + * ACPI_FLUSH_CPU_CACHE() flushes caches on entering sleep states.
> > > > + * It is required to prevent data loss.
> > > > + *
> > > > + * While running inside TDX guest, 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.
> > > > + *
> > > > + * TODO: Is it safe to generalize this from TDX guests to all guest kernels?
> > > > + */
> > > > +#define ACPI_FLUSH_CPU_CACHE()                                       \
> > > > +do {                                                         \
> > > > +     if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST))        \
> > > > +             wbinvd();                                       \
> > > > +} while (0)
> > >
> > > If it's safe, why not do it for all VMs?  Is there something specific
> > > about TDX which makes this more obviously known to be safe than for
> > > regular VMs?
> > >
> > > The patch description and the above comment make it sound like "we're
> > > not really sure this is safe, so we'll just use TDX as a testing ground
> > > for the idea." Which doesn't really inspire a lot of confidence in the
> > > stability of TD sleep states.
> > 
> > Agree, why is this marked as "TODO"? The cache flushes associated with
> > ACPI sleep states are to flush cache before bare metal power loss to
> > CPU caches and bare metal transition of DDR in self-refresh mode. If a
> > cache flush is required it is the responsibility of the hypervisor.
> > Either it is safe for all guests or it is unsafe for all guests, not
> > TD specific.
> 
> Do we have "any VM" check? I can't find it right away.

X86_FEATURE_HYPERVISOR

-- 
Josh