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* [PATCH v2] KVM: arm64: Skip more of the SError vaxorcism
@ 2019-06-10 16:30 James Morse
  2019-06-10 16:38 ` Robin Murphy
  2019-06-10 16:58 ` Marc Zyngier
  0 siblings, 2 replies; 5+ messages in thread
From: James Morse @ 2019-06-10 16:30 UTC (permalink / raw)
  To: linux-arm-kernel, kvmarm; +Cc: Marc Zyngier, Catalin Marinas, Will Deacon

During __guest_exit() we need to consume any SError left pending by the
guest so it doesn't contaminate the host. With v8.2 we use the
ESB-instruction. For systems without v8.2, we use dsb+isb and unmask
SError. We do this on every guest exit.

Use the same dsb+isr_el1 trick, this lets us know if an SError is pending
after the dsb, allowing us to skip the isb and self-synchronising PSTATE
write if its not.

This means SError remains masked during KVM's world-switch, so any SError
that occurs during this time is reported by the host, instead of causing
a hyp-panic.

If you give gcc likely()/unlikely() hints in an if() condition, it
shuffles the generated assembly so that the likely case is immediately
after the branch. Lets do the same here.

Signed-off-by: James Morse <james.morse@arm.com>
---
This patch was previously posted as part of:
[v1] https://lore.kernel.org/linux-arm-kernel/20190604144551.188107-1-james.morse@arm.com/

 arch/arm64/kvm/hyp/entry.S | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index a5a4254314a1..c2de1a1faaf4 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -161,18 +161,24 @@ alternative_if ARM64_HAS_RAS_EXTN
 	orr	x0, x0, #(1<<ARM_EXIT_WITH_SERROR_BIT)
 1:	ret
 alternative_else
-	// If we have a pending asynchronous abort, now is the
-	// time to find out. From your VAXorcist book, page 666:
+	dsb	sy		// Synchronize against in-flight ld/st
+	mrs	x2, isr_el1
+	and	x2, x2, #(1<<8)	// ISR_EL1.A
+	cbnz	x2, 2f
+	ret
+
+2:
+	// We know we have a pending asynchronous abort, now is the
+	// time to flush it out. From your VAXorcist book, page 666:
 	// "Threaten me not, oh Evil one!  For I speak with
 	// the power of DEC, and I command thee to show thyself!"
 	mrs	x2, elr_el2
+alternative_endif
 	mrs	x3, esr_el2
 	mrs	x4, spsr_el2
 	mov	x5, x0
 
-	dsb	sy		// Synchronize against in-flight ld/st
 	msr	daifclr, #4	// Unmask aborts
-alternative_endif
 
 	// This is our single instruction exception window. A pending
 	// SError is guaranteed to occur at the earliest when we unmask
-- 
2.20.1

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-- links below jump to the message on this page --
2019-06-10 16:30 [PATCH v2] KVM: arm64: Skip more of the SError vaxorcism James Morse
2019-06-10 16:38 ` Robin Murphy
2019-06-18 15:04   ` James Morse
2019-06-10 16:58 ` Marc Zyngier
2019-06-18 15:05   ` James Morse

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