[PATCH v2] target/arm/kvm: Retry KVM_CREATE_VM call if it fails EINTR

[PATCH v2] target/arm/kvm: Retry KVM_CREATE_VM call if it fails EINTR

[Peter Maydell]

Occasionally the KVM_CREATE_VM ioctl can return EINTR, even though
there is no pending signal to be taken. In commit 94ccff13382055
we added a retry-on-EINTR loop to the KVM_CREATE_VM call in the
generic KVM code. Adopt the same approach for the use of the
ioctl in the Arm-specific KVM code (where we use it to create a
scratch VM for probing for various things).

For more information, see the mailing list thread:
https://lore.kernel.org/qemu-devel/8735e0s1zw.wl-maz@kernel.org/

Reported-by: Vitaly Chikunov <vt@altlinux.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
The view in the thread seems to be that this is a kernel bug (because
in QEMU's case there shouldn't be a signal to be delivered at this
point because of our signal handling strategy); so I've adopted the
same "just retry-on-EINTR for this specific ioctl" approach that
commit 94ccff13 did, rather than, for instance, something wider like
"make kvm_ioctl() and friends always retry on EINTR".

v2: correctly check for -1 and errno is EINTR...
---
 target/arm/kvm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index e5c1bd50d29..356199c9e25 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -79,7 +79,9 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
     if (max_vm_pa_size < 0) {
         max_vm_pa_size = 0;
     }
-    vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
+    do {
+        vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
+    } while (vmfd == -1 && errno == -EINTR);
     if (vmfd < 0) {
         goto err;
     }
-- 
2.25.1

Re: [PATCH v2] target/arm/kvm: Retry KVM_CREATE_VM call if it fails EINTR

[Eric Auger]

Hi Peter,

On 9/27/22 18:49, Peter Maydell wrote:
> Occasionally the KVM_CREATE_VM ioctl can return EINTR, even though
> there is no pending signal to be taken. In commit 94ccff13382055
> we added a retry-on-EINTR loop to the KVM_CREATE_VM call in the
> generic KVM code. Adopt the same approach for the use of the
> ioctl in the Arm-specific KVM code (where we use it to create a
> scratch VM for probing for various things).
>
> For more information, see the mailing list thread:
> https://lore.kernel.org/qemu-devel/8735e0s1zw.wl-maz@kernel.org/
>
> Reported-by: Vitaly Chikunov <vt@altlinux.org>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> The view in the thread seems to be that this is a kernel bug (because
> in QEMU's case there shouldn't be a signal to be delivered at this
> point because of our signal handling strategy); so I've adopted the
> same "just retry-on-EINTR for this specific ioctl" approach that
> commit 94ccff13 did, rather than, for instance, something wider like
> "make kvm_ioctl() and friends always retry on EINTR".
>
> v2: correctly check for -1 and errno is EINTR...
> ---
>  target/arm/kvm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index e5c1bd50d29..356199c9e25 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -79,7 +79,9 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
>      if (max_vm_pa_size < 0) {
>          max_vm_pa_size = 0;
>      }
> -    vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
> +    do {
> +        vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
> +    } while (vmfd == -1 && errno == -EINTR);
shouldn't it be errno == EINTR?

Eric
>      if (vmfd < 0) {
>          goto err;
>      }

Re: [PATCH v2] target/arm/kvm: Retry KVM_CREATE_VM call if it fails EINTR

[Peter Maydell]

On Tue, 27 Sept 2022 at 18:07, Eric Auger <eric.auger@redhat.com> wrote:
>
> Hi Peter,
>
> On 9/27/22 18:49, Peter Maydell wrote:
> > Occasionally the KVM_CREATE_VM ioctl can return EINTR, even though
> > there is no pending signal to be taken. In commit 94ccff13382055
> > we added a retry-on-EINTR loop to the KVM_CREATE_VM call in the
> > generic KVM code. Adopt the same approach for the use of the
> > ioctl in the Arm-specific KVM code (where we use it to create a
> > scratch VM for probing for various things).
> >
> > For more information, see the mailing list thread:
> > https://lore.kernel.org/qemu-devel/8735e0s1zw.wl-maz@kernel.org/
> >
> > Reported-by: Vitaly Chikunov <vt@altlinux.org>
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> > The view in the thread seems to be that this is a kernel bug (because
> > in QEMU's case there shouldn't be a signal to be delivered at this
> > point because of our signal handling strategy); so I've adopted the
> > same "just retry-on-EINTR for this specific ioctl" approach that
> > commit 94ccff13 did, rather than, for instance, something wider like
> > "make kvm_ioctl() and friends always retry on EINTR".
> >
> > v2: correctly check for -1 and errno is EINTR...
> > ---
> >  target/arm/kvm.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> > index e5c1bd50d29..356199c9e25 100644
> > --- a/target/arm/kvm.c
> > +++ b/target/arm/kvm.c
> > @@ -79,7 +79,9 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
> >      if (max_vm_pa_size < 0) {
> >          max_vm_pa_size = 0;
> >      }
> > -    vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
> > +    do {
> > +        vmfd = ioctl(kvmfd, KVM_CREATE_VM, max_vm_pa_size);
> > +    } while (vmfd == -1 && errno == -EINTR);
> shouldn't it be errno == EINTR?

Augh. Yes.

-- PMM