[Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Pavel Fedin]

 This small patch fixes a long-standing bug: A15 model is used by default
also on ARM64 target. As a result, attempt to run the VM without explicit
-cpu specification does not do anything good.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
---
 hw/arm/virt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 6a45d94..b064bac 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -785,7 +785,11 @@ static void machvirt_init(MachineState *machine)
     char **cpustr;
 
     if (!cpu_model) {
+#ifdef TARGET_AARCH64
+        cpu_model = "cortex-a57";
+#else
         cpu_model = "cortex-a15";
+#endif
     }
 
     /* Separate the actual CPU model name from any appended features */
-- 
1.9.5.msysgit.0

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Peter Maydell]

On 6 May 2015 at 16:03, Pavel Fedin <p.fedin@samsung.com> wrote:
>  This small patch fixes a long-standing bug: A15 model is used by default
> also on ARM64 target. As a result, attempt to run the VM without explicit
> -cpu specification does not do anything good.
>
> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
> ---
>  hw/arm/virt.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 6a45d94..b064bac 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -785,7 +785,11 @@ static void machvirt_init(MachineState *machine)
>      char **cpustr;
>
>      if (!cpu_model) {
> +#ifdef TARGET_AARCH64
> +        cpu_model = "cortex-a57";
> +#else
>          cpu_model = "cortex-a15";
> +#endif
>      }

I'm afraid this is wrong -- the same command line on
qemu-system-arm and qemu-system-aarch64 should do the same thing.

I agree that the current behaviour is confusing but I don't think
this is the correct way to fix it.

-- PMM

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Pavel Fedin]

 Hello!

> I'm afraid this is wrong -- the same command line on qemu-system-arm and
> qemu-system-aarch64 should do the same thing.

 But 64-bit A15 is nonsense, because it's 32-bit CPU, isn't it?

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Peter Maydell]

On 6 May 2015 at 19:06, Pavel Fedin <p.fedin@samsung.com> wrote:
>  Hello!
>
>> I'm afraid this is wrong -- the same command line on qemu-system-arm and
>> qemu-system-aarch64 should do the same thing.
>
>  But 64-bit A15 is nonsense, because it's 32-bit CPU, isn't it?

Yes. qemu-system-aarch64 -cpu cortex-a15 gets you a 32-bit
Cortex-A15. This is exactly like the x86 QEMU, where you can
emulate all the 32-bit x86 CPUs in qemu-system-x86_64.

-- PMM

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Pavel Fedin]

 Hello!

> Yes. qemu-system-aarch64 -cpu cortex-a15 gets you a 32-bit Cortex-A15. This is
> exactly like the x86 QEMU, where you can emulate all the 32-bit x86 CPUs in
> qemu-system-x86_64.

 And what is the default in x86_64 ? I believe it's 64-bit CPU, and not i686, isn't it ?
 I agree that -cpu cortex-a15 might work with qemu-system-aarch64. But isn't it expected to get the "native" CPU by default ? This behavior of qemu causes weird thing with libvirt/virt-manager. I create a VM, specify "aarch64" as architecture, and set "hypervisor default" as CPU model. With "hypervisor default" libvirt does not pass any -cpu option to qemu, leaving it up to the emulator (exactly as expected). So, i supply 64-bit kernel and rootfs, and nothing works. I have to explicitly change the CPU to "cortex-a57". Isn't it confusing to have such a default ?
 This is even more confusing since currently i don't know any implementation of aarch64 except A57 and A53 (which are the same from software point of view). So, i believe, A57 is quite a reasonable default.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Cole Robinson]

On 05/07/2015 02:36 AM, Pavel Fedin wrote:
>  Hello!
> 
>> Yes. qemu-system-aarch64 -cpu cortex-a15 gets you a 32-bit Cortex-A15. This is
>> exactly like the x86 QEMU, where you can emulate all the 32-bit x86 CPUs in
>> qemu-system-x86_64.
> 
>  And what is the default in x86_64 ? I believe it's 64-bit CPU, and not i686, isn't it ?
>  I agree that -cpu cortex-a15 might work with qemu-system-aarch64. But isn't it expected to get the "native" CPU by default ? This behavior of qemu causes weird thing with libvirt/virt-manager. I create a VM, specify "aarch64" as architecture, and set "hypervisor default" as CPU model. With "hypervisor default" libvirt does not pass any -cpu option to qemu, leaving it up to the emulator (exactly as expected). So, i supply 64-bit kernel and rootfs, and nothing works. I have to explicitly change the CPU to "cortex-a57". Isn't it confusing to have such a default ?
>  This is even more confusing since currently i don't know any implementation of aarch64 except A57 and A53 (which are the same from software point of view). So, i believe, A57 is quite a reasonable default.
> 

FWIW virt-manager 1.2.0 (just released) will do the following when creating a
new VM:

- aarch64 + kvm : -cpu host
- aarch64 + tcg : -cpu cortex-a57
- arm32 + kvm : -cpu host
- arm32 + tcg : defer to qemu

Though if you explicitly request 'hypervisor default' then we won't specify
any -cpu and defer to qemu, which will hit the cortex-a15 default for aarch64

- Cole

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Pavel Fedin]

 Hello!

> FWIW virt-manager 1.2.0 (just released) will do the following when creating a
> new VM:
> 
> - aarch64 + kvm : -cpu host
> - aarch64 + tcg : -cpu cortex-a57
> - arm32 + kvm : -cpu host
> - arm32 + tcg : defer to qemu
> 
> Though if you explicitly request 'hypervisor default' then we won't specify
> any -cpu and defer to qemu, which will hit the cortex-a15 default for aarch64

 virt-manager is not the only tool to create VMs...
 But, okay. Seems you just don't want to change this. Well... I still don't agree and this
default looks strange for me, but it's just me. I'm out of further arguments.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Peter Crosthwaite]

On Fri, May 8, 2015 at 12:14 AM, Pavel Fedin <p.fedin@samsung.com> wrote:
>  Hello!
>
>> FWIW virt-manager 1.2.0 (just released) will do the following when creating a
>> new VM:
>>
>> - aarch64 + kvm : -cpu host
>> - aarch64 + tcg : -cpu cortex-a57
>> - arm32 + kvm : -cpu host
>> - arm32 + tcg : defer to qemu
>>
>> Though if you explicitly request 'hypervisor default' then we won't specify
>> any -cpu and defer to qemu, which will hit the cortex-a15 default for aarch64
>
>  virt-manager is not the only tool to create VMs...
>  But, okay. Seems you just don't want to change this. Well... I still don't agree and this
> default looks strange for me, but it's just me. I'm out of further arguments.
>

I think the real bug is the naming scheme for QEMU targets. QEMU
targets such as Aarch64 and x86_64 are following the long established
convention of using an ISA for a name, but are ultimately badly named
as they support multiple ISAs beyond what their singular name
suggests.

"qemu-system-arm" would be the be the best name for the
works-for-all-ARM QEMU but that name is taken. But if we are true to
qemu-system-aarch64 being a functional superset of arm, can we just
unify as the one arch ("arm") and phase out qemu-system-aarch64 (would
become an alias of qemu-system-arm)?. Linux user builds are the main
complication, but that is the case where it very much does make sense
to tie to an ISA. Ignoring legacy, the ideal naming scheme would be:

qemu-system-arm
aarch64-linux-user
aarch32-linux-user

Regards,
Peter

Regards,
Peter

> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia
>
>
>

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Paolo Bonzini]

On 08/05/2015 09:14, Pavel Fedin wrote:
>  Hello!
> 
>> FWIW virt-manager 1.2.0 (just released) will do the following when creating a
>> new VM:
>>
>> - aarch64 + kvm : -cpu host
>> - aarch64 + tcg : -cpu cortex-a57
>> - arm32 + kvm : -cpu host
>> - arm32 + tcg : defer to qemu
>>
>> Though if you explicitly request 'hypervisor default' then we won't specify
>> any -cpu and defer to qemu, which will hit the cortex-a15 default for aarch64
> 
>  virt-manager is not the only tool to create VMs...
>  But, okay. Seems you just don't want to change this. Well... I still don't agree and this
> default looks strange for me, but it's just me. I'm out of further arguments.

Actually I agree with your patch.  It's the same that x86 does.

Paolo

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Pavel Fedin]

 Hello!

> Ignoring legacy, the ideal naming scheme would be:
> 
> qemu-system-arm
> aarch64-linux-user
> aarch32-linux-user

 You know, i rethought. Actually, the question concerns only "virt" machine, because for other real machines the CPU should be the same as on real HW, and this is no question. Shouldn't our default for "virt" be just "host" ? This would eliminate all ambiguity i believe...

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Peter Crosthwaite]

On Fri, May 8, 2015 at 1:44 AM, Pavel Fedin <p.fedin@samsung.com> wrote:
>  Hello!
>
>> Ignoring legacy, the ideal naming scheme would be:
>>
>> qemu-system-arm
>> aarch64-linux-user
>> aarch32-linux-user
>
>  You know, i rethought. Actually, the question concerns only "virt" machine, because for other real machines the CPU should be the same as on real HW, and this is no question. Shouldn't our default for "virt" be just "host" ? This would eliminate all ambiguity i believe...
>

What's it then meant to do for TCG (where the host could be anything)?

And I think this is still an issue as it will be a functional diff
between qemu-system-arm and qemu-system-aarch64 breaking compatability
between the too and forever committing us to have to maintain two
builds.

Regards,
Peter

> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia
>
>
>

Re: [Qemu-devel] [PATCH] Fix default CPU model for ARM64

[Cole Robinson]

On 05/08/2015 03:14 AM, Pavel Fedin wrote:
>  Hello!
> 
>> FWIW virt-manager 1.2.0 (just released) will do the following when creating a
>> new VM:
>>
>> - aarch64 + kvm : -cpu host
>> - aarch64 + tcg : -cpu cortex-a57
>> - arm32 + kvm : -cpu host
>> - arm32 + tcg : defer to qemu
>>
>> Though if you explicitly request 'hypervisor default' then we won't specify
>> any -cpu and defer to qemu, which will hit the cortex-a15 default for aarch64
> 
>  virt-manager is not the only tool to create VMs...
>  But, okay. Seems you just don't want to change this. Well... I still don't agree and this
> default looks strange for me, but it's just me. I'm out of further arguments.
> 

I wasn't implying any opinion about your proposed patch, just clarifying the
details about how latest virt-manager performs since you mentioned it
upthread. FWIW I agree with the idea of your patch

- Cole