Re: [PATCH v21 13/15] KVM: selftests: arm64: Only restore SPSR_EL1 and ELR_EL1 if they change
flat view
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: 2026-10-01 16:41:55
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kvmarm, linux-doc, linux-kselftest, lkml
On Wed, Sep 30, 2026 at 10:48:23PM +0100, Mark Brown wrote:
When EXLOCK is enabled for ELx writes to SPSR_ELx and ELR_ELx generate GCS exceptions. Currently the selftest helpers unconditionally reload the saved values for these registers regardless of if the handler didn't change them. This gets in the way of EXLOCK testing, making it hard to generate contexts which will generate an invalid ERET since we never get as far as doing an exception return while EXLOCK is enabled.
Sounds reasonable.
In order to facilitate testing of nested virtualisation emulation of EXLOCK change our reloads into read/modify/write cycles, suppressing the writes when they would not change the value in the register.
Also sounds sensible.
Reviewed-by: Leonardo Bras <redacted> Signed-off-by: Mark Brown <broonie@kernel.org>
Also looks obviously correct (TM) to me so: Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
quoted hunk ↗ jump to hunk
--- tools/testing/selftests/kvm/lib/arm64/handlers.S | 10 ++++++++++ 1 file changed, 10 insertions(+)diff --git a/tools/testing/selftests/kvm/lib/arm64/handlers.S b/tools/testing/selftests/kvm/lib/arm64/handlers.S index 0e443eadfac6..367c22d61cba 100644 --- a/tools/testing/selftests/kvm/lib/arm64/handlers.S +++ b/tools/testing/selftests/kvm/lib/arm64/handlers.S@@ -33,8 +33,18 @@ .macro restore_registers ldp x1, x2, [sp, #16 * 16] /* PC, PSTATE */ + + /* Suppress redundant writes to support GCS exception lock testing */ + mrs x3, elr_el1 + cmp x1, x3 + beq .Lskip_elr_\@ msr elr_el1, x1 +.Lskip_elr_\@: + mrs x3, spsr_el1 + cmp x2, x3 + beq .Lskip_spsr_\@
Even as an arm64 assembly novice I can see what this is doing :)
msr spsr_el1, x2 +.Lskip_spsr_\@: /* sp is not restored */ ldp x30, xzr, [sp, #16 * 15] /* x30, SP */ -- 2.47.3
-- Cheers, Lorenzo