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

[Anshuman Khandual]

On 12/11/23 11:30, Anshuman Khandual wrote:
> On 12/4/23 14:12, Marc Zyngier wrote:
>> On Fri, 01 Dec 2023 05:39:03 +0000,
>> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>> 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();
	....
}