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SubjectRe: [PATCH 2/2] KVM: arm64: selftests: Disable single-step without relying on ucall()
On Thu, Nov 17, 2022 at 12:23:50AM +0000, Sean Christopherson wrote:
> Automatically disable single-step when the guest reaches the end of the
> verified section instead of using an explicit ucall() to ask userspace to
> disable single-step. An upcoming change to implement a pool-based scheme
> for ucall() will add an atomic operation (bit test and set) in the guest
> ucall code, and if the compiler generate "old school" atomics, e.g.

Off topic, but I didn't ask when we were discussing this issue. What is
the atomic used for in the pool-based ucall implementation?

> 40e57c: c85f7c20 ldxr x0, [x1]
> 40e580: aa100011 orr x17, x0, x16
> 40e584: c80ffc31 stlxr w15, x17, [x1]
> 40e588: 35ffffaf cbnz w15, 40e57c <__aarch64_ldset8_sync+0x1c>
>
> the guest will hang as the local exclusive monitor is reset by eret,
> i.e. the stlxr will always fail due to the VM-Exit for the debug
> exception.

... due to the debug exception taken to EL2.

What's a VM-Exit anyways? ;-)

> Link: https://lore.kernel.org/all/20221006003409.649993-8-seanjc@google.com
> Cc: Oliver Upton <oliver.upton@linux.dev>
> Cc: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

With the commit message nit:

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

--
Thanks,
Oliver

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