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

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