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KVM: arm64: Skip more of the SError vaxorcism
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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.

As we're benchmarking this code lets polish the layout. 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>

Changes since v2:
 * Added isb after the dsb to prevent an early read

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
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James Morse authored and Marc Zyngier committed Jul 5, 2019
1 parent dad6321 commit 11b4162
Showing 1 changed file with 10 additions and 4 deletions.
14 changes: 10 additions & 4 deletions arch/arm64/kvm/hyp/entry.S
Original file line number Diff line number Diff line change
Expand Up @@ -151,18 +151,24 @@ 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
isb // Prevent an early read of side-effect free ISR
mrs x2, isr_el1
tbnz x2, #8, 2f // ISR_EL1.A
ret
nop
2:
alternative_endif
// 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
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
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