Re: [PULL 3/9] meson: Disallow 64-bit on 32-bit Xen emulation

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

+Vikram

On 18/2/25 15:10, Andrew Cooper wrote:
> On 18/02/2025 11:20 am, Philippe Mathieu-Daudé wrote:
>> Hi,
>>
>> Adding Xen community.
>>
>> On 8/2/25 21:57, Richard Henderson wrote:
>>> Require a 64-bit host binary to spawn a 64-bit guest.
>>>
>>> Reviewed-by: Thomas Huth <thuth@redhat.com>
>>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>>> ---
>>>    meson.build | 9 +++++++--
>>>    1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/meson.build b/meson.build
>>> index 1af8aeb194..911955cfa8 100644
>>> --- a/meson.build
>>> +++ b/meson.build
>>> @@ -304,9 +304,14 @@ else
>>>    endif
>>>    accelerator_targets = { 'CONFIG_KVM': kvm_targets }
>>>    -if cpu in ['x86', 'x86_64']
>>> +if cpu == 'x86'
>>> +  xen_targets = ['i386-softmmu']
>>> +elif cpu == 'x86_64'
>>>      xen_targets = ['i386-softmmu', 'x86_64-softmmu']
>>> -elif cpu in ['arm', 'aarch64']
>>> +elif cpu == 'arm'
>>> +  # i386 emulator provides xenpv machine type for multiple
>>> architectures
>>> +  xen_targets = ['i386-softmmu']
>>
>> Is actually someone *testing* this config? I'm having hard time building
>> it, so am very suspicious about how it runs, and start to wonder if I'm
>> not just wasting my time (as could be our CI).
> 
> It was intentional.  I believe it was Stefano (CC'd) who introduced it.

In the introduction commit, "ARM targets" is used, so apparently both
32/64bit were picked deliberately:

---
commit aaea616d54317b8a0154adf52303a51da2d8d56f
Author: Vikram Garhwal <vikram.garhwal@amd.com>
Date:   Wed Jun 14 17:03:38 2023 -0700

     meson.build: enable xenpv machine build for ARM

     Add CONFIG_XEN for aarch64 device to support build for ARM targets.

     Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
     Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
     Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

diff --git a/meson.build b/meson.build
index 481865bfa97..cfa98e9e25f 100644
--- a/meson.build
+++ b/meson.build
@@ -136,7 +136,7 @@ endif
  if cpu in ['x86', 'x86_64', 'arm', 'aarch64']
    # i386 emulator provides xenpv machine type for multiple architectures
    accelerator_targets += {
-    'CONFIG_XEN': ['i386-softmmu', 'x86_64-softmmu'],
+    'CONFIG_XEN': ['i386-softmmu', 'x86_64-softmmu', 'aarch64-softmmu'],
    }
  endif
  if cpu in ['x86', 'x86_64']
---

> Xen uses qemu-system-i386 everywhere because qemu-system-x86_64 doesn't
> make compatible VMs.  I'm not sure why; I suspect it's bugs in the Xen
> machine types, but I don't know QEMU well enough to be sure.
> 
> Another thing that (at least, was) tied to qemu-system-i386 was using
> Qemu as a XenBlk <-> QCOW adapter, at which point it wasn't even really
> a system emulator, just a paravirtual disk implementation.
> 
> This is, AIUI, what ARM wants with the xenpv machine.  If there's a
> better way to do this, please do say.

No, I concur.

> Looking through Xen's CI, I can't see any of the ARM builds building
> QEMU at all.  I think it's quite possible it's not tested any more.

We only cross-build, see our cross-arm64-xen-only job:
https://gitlab.com/qemu-project/qemu/-/jobs/9165958873

Note, if it is not clear, the problem I have is to test Xen on
32-bit ARM hosts; I don't have any problem with 64-bit ones.

Regards,

Phil.