linux-kernel - Re: [PATCH v19 10/11] KVM: arm64: nvhe: Disable branch generation in nVHE guests

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Message-ID: <CAL_JsqLG4gu6c6=x_wG6XT0WaCC_ahH5eWHk3K9RcF0ZQrDR=A@mail.gmail.com>
Date: Thu, 13 Feb 2025 17:16:45 -0600
From: Rob Herring <robh@...nel.org>
To: Leo Yan <leo.yan@....com>
Cc: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Catalin Marinas <catalin.marinas@....com>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	James Clark <james.clark@...aro.org>, Anshuman Khandual <anshuman.khandual@....com>, 
	linux-arm-kernel@...ts.infradead.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v19 10/11] KVM: arm64: nvhe: Disable branch generation in
 nVHE guests

On Thu, Feb 13, 2025 at 11:03 AM Leo Yan <leo.yan@....com> wrote:
>
> On Sun, Feb 02, 2025 at 06:43:04PM -0600, Rob Herring (Arm) wrote:
>
> [...]
>
> > +static void __debug_save_brbe(u64 *brbcr_el1)
> > +{
> > +       *brbcr_el1 = 0;
> > +
> > +       /* Check if the BRBE is enabled */
> > +       if (!(read_sysreg_el1(SYS_BRBCR) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
> > +               return;
> > +
> > +       /*
> > +        * Prohibit branch record generation while we are in guest.
> > +        * Since access to BRBCR_EL1 is trapped, the guest can't
> > +        * modify the filtering set by the host.
> > +        */
> > +       *brbcr_el1 = read_sysreg_el1(SYS_BRBCR);
> > +       write_sysreg_el1(0, SYS_BRBCR);
> > +}
>
> Should flush branch record and use isb() before exit host kernel?

I don't think so. The isb()'s in the other cases appear to be related
to ordering WRT memory buffers. BRBE is just registers. I would assume
that there's some barrier before we switch to the guest.

> I see inconsistence between the function above and BRBE's disable
> function. Here it clears E0BRE / ExBRE bits for disabling BRBE, but the
> BRBE driver sets the PAUSED bit in BRBFCR_EL1 for disabling BRBE.

Indeed. This works, but the enabled check won't work. I'm going to add
clearing BRBCR to brbe_disable(), and this part will stay the same.

Rob