Re: [PATCH V15 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests

On 12/11/23 11:30, Anshuman Khandual wrote:

On 12/4/23 14:12, Marc Zyngier wrote:

quoted

On Fri, 01 Dec 2023 05:39:03 +0000,
Anshuman Khandual [off-list ref] wrote:

quoted

Disable the BRBE before we enter the guest, saving the status and enable it
back once we get out of the guest. This is just to avoid capturing records
in the guest kernel/userspace, which would be confusing the samples.

Why does it have to be limited to non-VHE? What protects host EL0
records from guest's EL0 execution when the host is VHE?

In a scenario when running the host in VHE mode
 
- The host might have enabled branch records for host EL0 through BRBCR_EL2.E0HBRE
  indirectly via accessing BRBCR_EL1.E0BRE

- But after the guest switches in on the cpu - BRBCR_EL2.E0HBRE will still remain
  set and enable branch records in guest EL0 as well because BRBCR_EL1.E0BRE will
  not have any effect when EL2 is implemented and HCR_EL2.TGE == 1. The guest EL0
  execution branch records will find their way into branch records being captured
  for host EL0

You are right. The host EL0 branch records too need to be protected from guest EL0
execution. A similar BRBCR_EL1 save/restore mechanism is needed for VHE as well ?

Looking at this again, seems like host EL0 records will be protected from
guest EL0 execution as HCR_EL2.TGE toggles when th guest switches in thus
enforcing BRBCR_EL1.E0BRE (which is clear) requirement for capturing guest
EL0 branch records.

arch/arm64/kvm/hyp/vhe/tlb.c

__tlb_switch_to_guest() {
	....
	val = read_sysreg(hcr_el2);
        val &= ~HCR_TGE;
        write_sysreg(val, hcr_el2);
	isb();
}

HCR_TGE comes back via HCR_HOST_VHE_FLAGS when the host switches back in.

__tlb_switch_to_host() {
	write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
        isb();
	....
}

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