Re: [PATCH 2/8] KVM: arm64: Synchronise HCR_EL2 writes on the guest exit path

Fuad Tabba
2026-04-28T14:22:36Z

Hi Will,

On Tue, 28 Apr 2026 at 14:50, Will Deacon <will@kernel.org> wrote:
>
> On Tue, Apr 28, 2026 at 11:30:02AM +0100, Fuad Tabba wrote:
> > MSR HCR_EL2 is not self-synchronising. Per ARM DDI 0487 M.b K1.2.4
> > (p.K1-16823) and B2.6.1 (p.B2-297), a Context Synchronisation Event
> > is required between an HCR_EL2 write and any subsequent direct
> > register access at the same EL that depends on the new value being
> > in effect.
> >
> > On the entry path, the HCR_EL2 write in __activate_traps is followed
> > by further EL2 sysreg work (MDCR_EL2, CPTR_EL2, VBAR_EL2, and on the
> > speculative-AT errata path SCTLR_EL1/TCR_EL1) before ERET into the
> > guest. None of those intervening accesses depend on the new HCR_EL2
> > value, and ERET is a CSE per ARM DDI 0487 M.b D1.4.4.1 rule RBWCFK
> > (p. D1-7209) conditional on SCTLR_EL2.EOS=1, which is set
> > unconditionally by INIT_SCTLR_EL2_MMU_ON (see the prerequisite patch
> > in this series). The requirement is therefore satisfied implicitly
> > on the activate path.
> >
> > The deactivate path is different: after write_sysreg_hcr() in
> > __deactivate_traps() further EL2 sysreg work runs before any natural
> > CSE - on nVHE, __deactivate_cptr_traps and the VBAR_EL2 write; on
> > VHE, the timer context save which reads CNTP_CVAL_EL0 under the new
> > TGE/E2H, and the EL1 sysreg restore. Add an explicit isb() at each
> > of the two deactivate sites.
> >
> > The practical impact today is bounded: HCR_EL2.E2H does not toggle
> > in either path, and the trap bits being changed primarily affect
> > EL1&0 behaviour. But the architectural rule should be honoured.
> > Note that write_sysreg_hcr() itself already issues isb() on the
> > Ampere errata path (sysreg.h), confirming the architectural
> > expectation; the fast path optimises that away.
> >
> > The fix is at the call sites rather than inside write_sysreg_hcr()
> > because the macro has many users (e.g. the activate path, at.c,
> > hardirq.h, ptrauth alternatives) where the immediately-following
> > code either reaches ERET or has another CSE; making the macro emit
> > an unconditional ISB would impose unnecessary cost on those
> > well-formed users.
> >
> > Fixes: 9404673293b0 ("KVM: arm64: timers: Correctly handle TGE flip with CNTPOFF_EL2")
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> > arch/arm64/kvm/hyp/nvhe/switch.c | 11 +++++++++++
> > arch/arm64/kvm/hyp/vhe/switch.c | 11 +++++++++++
> > 2 files changed, 22 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> > index 8d1df3d33595..9d7ead5a5503 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> > @@ -105,6 +105,17 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
> > __deactivate_traps_common(vcpu);
> >
> > write_sysreg_hcr(this_cpu_ptr(&kvm_init_params)->hcr_el2);
> > + /*
> > + * MSR HCR_EL2 is not self-synchronising. Per ARM ARM K1.2.4 p.K1-16823
> > + * and B2.6.1 p.B2-297, a Context Synchronisation Event is required
> > + * between an HCR_EL2 write and any subsequent direct register access at
> > + * the same EL that depends on the new value being in effect.
> > + * The activate_traps path falls through to ERET (a CSE), but the
> > + * deactivate path still executes further EL2 sysreg work (CPTR/VBAR
> > + * writes below) before any natural CSE, so make the synchronisation
> > + * explicit.
> > + */
> > + isb();
>
> Sorry, but I don't understand this. Please can you explain why you think
> that CPTR and VBAR have an ordering dependency on HCR_EL2? Preferably,
> the comment would talk about the specific fields that are relevant.

You're right, I misread the spec in my excitement to fix an issue
raised by my new and improved prompts. I went back to the access
pseudocode for the registers being written after write_sysreg_hcr():

- CPTR_EL2/VBAR_EL2 (nVHE) are direct EL2-only registers; their write
semantics at EL2 don't consult HCR_EL2.
- CPACR_EL1/VBAR_EL1/CNTP_CVAL_EL0 (VHE) redirect to their EL2
counterparts on ELIsInHost(EL2) — i.e., HCR_EL2.E2H — and E2H doesn't
toggle here. TGE isn't a condition for these EL2-level accesses.

So the whole "depends on the new value being in effect" doesn't
actually bite for any of the accesses I cited. I'll drop this patch.
The natural CSEs cover whatever real synchronisation is needed.

Cheers,
/fuad

>
> Will