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

[Rob Herring]

On Thu, Feb 13, 2025 at 11:03 AM Leo Yan <leo.yan at arm.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