Hi Drew,
On 21/08/2019 20:50, Andrew Jones wrote:
> If after an MMIO exit to userspace a VCPU is immediately run with an
> immediate_exit request, such as when a signal is delivered or an MMIO
> emulation completion is needed, then the VCPU completes the MMIO
> emulation and immediately returns to userspace. As the exit_reason
> does not get changed from KVM_EXIT_MMIO in these cases we have to
> be careful not to complete the MMIO emulation again, when the VCPU is
> eventually run again, because the emulation does an instruction skip
> (and doing too many skips would be a waste of guest code :-) We need
> to use additional VCPU state to track if the emulation is complete.
> As luck would have it, we already have 'mmio_needed', which even
> appears to be used in this way by other architectures already.
>
> Fixes: 0d640732dbeb ("arm64: KVM: Skip MMIO insn after emulation")
> Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx>
> ---
> virt/kvm/arm/arm.c | 3 ++-
> virt/kvm/arm/mmio.c | 1 +
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 35a069815baf..322cf9030bbe 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -669,7 +669,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> if (ret)
> return ret;
>
> - if (run->exit_reason == KVM_EXIT_MMIO) {
> + if (vcpu->mmio_needed) {
> + vcpu->mmio_needed = 0;
> ret = kvm_handle_mmio_return(vcpu, vcpu->run);
> if (ret)
> return ret;
> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> index a8a6a0c883f1..2d9b5e064ae0 100644
> --- a/virt/kvm/arm/mmio.c
> +++ b/virt/kvm/arm/mmio.c
> @@ -201,6 +201,7 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
> if (is_write)
> memcpy(run->mmio.data, data_buf, len);
> vcpu->stat.mmio_exit_user++;
> + vcpu->mmio_needed = 1;
> run->exit_reason = KVM_EXIT_MMIO;
> return 0;
> }
>
Thanks for this. That's quite embarrassing. Out of curiosity,
how was this spotted?
Patch wise, I'd have a small preference for the following (untested)
patch, as it keeps the mmio_needed accesses close together, making
it easier to read (at least for me). What do you think?
M.
diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
index a8a6a0c883f1..6af5c91337f2 100644
--- a/virt/kvm/arm/mmio.c
+++ b/virt/kvm/arm/mmio.c
@@ -86,6 +86,12 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
unsigned int len;
int mask;
+ /* Detect an already handled MMIO return */
+ if (unlikely(!vcpu->mmio_needed))
+ return 0;
+
+ vcpu->mmio_needed = 0;
+
if (!run->mmio.is_write) {
len = run->mmio.len;
if (len > sizeof(unsigned long))
@@ -188,6 +194,7 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
run->mmio.is_write = is_write;
run->mmio.phys_addr = fault_ipa;
run->mmio.len = len;
+ vcpu->mmio_needed = 1;
if (!ret) {
/* We handled the access successfully in the kernel. */
--
Jazz is not dead, it just smells funny...
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