linux-kernel - Re: [PATCH v2 2/2] KVM: SEV: Prefer WBNOINVD over WBINVD for cache maintenance efficiency

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Message-ID: <brebuxyjirsfc257fpq4qxlowveolrabzetg2i3cj3ee6yzci3@j7zlc676gf7n>
Date: Tue, 14 Jan 2025 09:50:33 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	seanjc@...gle.com, pbonzini@...hat.com, kai.huang@...el.com, ubizjak@...il.com, 
	dave.jiang@...el.com, jgross@...e.com, kvm@...r.kernel.org, thomas.lendacky@....com, 
	pgonda@...gle.com, sidtelang@...gle.com, mizhang@...gle.com, rientjes@...gle.com, 
	szy0127@...u.edu.cn
Subject: Re: [PATCH v2 2/2] KVM: SEV: Prefer WBNOINVD over WBINVD for cache
 maintenance efficiency

On Mon, Jan 13, 2025 at 10:47:57AM -0800, Kevin Loughlin wrote:
> On Fri, Jan 10, 2025 at 12:23 AM Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >
> > On Thu, Jan 09, 2025 at 10:55:33PM +0000, Kevin Loughlin wrote:
> > > @@ -710,6 +711,14 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> > >       }
> > >  }
> > >
> > > +static void sev_wb_on_all_cpus(void)
> > > +{
> > > +     if (boot_cpu_has(X86_FEATURE_WBNOINVD))
> > > +             wbnoinvd_on_all_cpus();
> > > +     else
> > > +             wbinvd_on_all_cpus();
> >
> > I think the X86_FEATURE_WBNOINVD check should be inside wbnoinvd().
> > wbnoinvd() should fallback to WBINVD if the instruction is not supported
> > rather than trigger #UD.
> 
> I debated this as well and am open to doing it that way. One argument
> against silently falling back to WBINVD within wbnoinvd() (in general,
> not referring to this specific case) is that frequent WBINVD can cause
> soft lockups, whereas WBNOINVD is much less likely to do so. As such,
> there are potentially use cases where falling back to WBINVD would be
> undesirable (and would potentially be non-obvious behavior to the
> programmer calling wbnoinvd()), hence why I left the decision outside
> wbnoinvd().
> 
> That said, open to either way, especially since that "potential" use
> case doesn't apply here; just lemme know if you still have a strong
> preference for doing the check within wbnoinvd().

An alternative would be to fail wbnoinvd() with an error code and
possibly a WARN(). Crash on #UD is not helpful.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov