As feature flags are added or removed, the meanings of bits in the
`features` field can change between QEMU versions, causing migration
failures. Additionally, migrating the field is not useful because it is
a constant function of the CPU being used.

Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/machine.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/arm/machine.c b/target/arm/machine.c
index c9e9fd0a12..7f2511b6ed 100644
--- a/target/arm/machine.c
+++ b/target/arm/machine.c
@@ -834,7 +834,7 @@ const VMStateDescription vmstate_arm_cpu = {
         VMSTATE_UINT64(env.exclusive_addr, ARMCPU),
         VMSTATE_UINT64(env.exclusive_val, ARMCPU),
         VMSTATE_UINT64(env.exclusive_high, ARMCPU),
-        VMSTATE_UINT64(env.features, ARMCPU),
+        VMSTATE_UNUSED(sizeof(uint64_t)),
         VMSTATE_UINT32(env.exception.syndrome, ARMCPU),
         VMSTATE_UINT32(env.exception.fsr, ARMCPU),
         VMSTATE_UINT64(env.exception.vaddress, ARMCPU),
-- 
2.17.1

On Wed, Feb 03, 2021 at 11:13:40AM -0500, Aaron Lindsay wrote:
> As feature flags are added or removed, the meanings of bits in the
> `features` field can change between QEMU versions, causing migration
> failures. Additionally, migrating the field is not useful because it is
> a constant function of the CPU being used.
> 
> Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  target/arm/machine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/arm/machine.c b/target/arm/machine.c
> index c9e9fd0a12..7f2511b6ed 100644
> --- a/target/arm/machine.c
> +++ b/target/arm/machine.c
> @@ -834,7 +834,7 @@ const VMStateDescription vmstate_arm_cpu = {
>          VMSTATE_UINT64(env.exclusive_addr, ARMCPU),
>          VMSTATE_UINT64(env.exclusive_val, ARMCPU),
>          VMSTATE_UINT64(env.exclusive_high, ARMCPU),
> -        VMSTATE_UINT64(env.features, ARMCPU),
> +        VMSTATE_UNUSED(sizeof(uint64_t)),
>          VMSTATE_UINT32(env.exception.syndrome, ARMCPU),
>          VMSTATE_UINT32(env.exception.fsr, ARMCPU),
>          VMSTATE_UINT64(env.exception.vaddress, ARMCPU),
> -- 
> 2.17.1
>

I tested this with a KVM guest. Prior to this patch, a migration of a 4.1
guest from a 4.1 build of QEMU to a build of the latest bits of QEMU would
fail with a segmentation fault. With this patch, the migration succeeds.
However, migrating a virt-4.1 machine type from a QEMU of the latest bits
to a 4.1 build of QEMU still fails with

  qemu-system-aarch64: error while loading state for instance 0x0 of device 'pl011'
  qemu-system-aarch64: load of migration failed: No such file or directory

This means "ping-pong" migrations fail, which is another thing we would
prefer to work. But, as that appears to be a different bug, regarding this
patch

Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>

Thanks,
drew

On Wed, 3 Feb 2021 at 16:14, Aaron Lindsay <aaron@os.amperecomputing.com> wrote:
>
> As feature flags are added or removed, the meanings of bits in the
> `features` field can change between QEMU versions, causing migration
> failures. Additionally, migrating the field is not useful because it is
> a constant function of the CPU being used.
>
> Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  target/arm/machine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)



Applied to target-arm.next, thanks.

-- PMM

On 2/3/21 5:13 PM, Aaron Lindsay wrote:
> As feature flags are added or removed, the meanings of bits in the
> `features` field can change between QEMU versions, causing migration
> failures. Additionally, migrating the field is not useful because it is
> a constant function of the CPU being used.

Please don't bury patches within mailing list threads.

BTW you found yet another 13 years old problem :)
918f5dca18d ("target-arm: Extend feature flags to 64 bits")
aa941b94450 ("Savevm/loadvm bits for ARM core, the PXA2xx peripherals
and Spitz hardware.")

> 
> Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  target/arm/machine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/arm/machine.c b/target/arm/machine.c
> index c9e9fd0a12..7f2511b6ed 100644
> --- a/target/arm/machine.c
> +++ b/target/arm/machine.c
> @@ -834,7 +834,7 @@ const VMStateDescription vmstate_arm_cpu = {
>          VMSTATE_UINT64(env.exclusive_addr, ARMCPU),
>          VMSTATE_UINT64(env.exclusive_val, ARMCPU),
>          VMSTATE_UINT64(env.exclusive_high, ARMCPU),
> -        VMSTATE_UINT64(env.features, ARMCPU),
> +        VMSTATE_UNUSED(sizeof(uint64_t)),
>          VMSTATE_UINT32(env.exception.syndrome, ARMCPU),
>          VMSTATE_UINT32(env.exception.fsr, ARMCPU),
>          VMSTATE_UINT64(env.exception.vaddress, ARMCPU),
>