Messages in this thread | | | Date | Wed, 13 Dec 2023 10:26:16 +0530 | Subject | Re: [PATCH V15 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests | From | Anshuman Khandual <> |
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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@arm.com> 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(); .... }
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