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

[Leo Yan <leo.yan@arm.com>]

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 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.

Thanks,
Leo