Re: [PATCH 5/7] hw/virtio/virtio-mem: Convert VIRTIO_MEM_USABLE_EXTENT to runtime

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

On 7/3/25 17:38, Alex Bennée wrote:
> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> 
>> Use qemu_arch_available() to check at runtime if a target
>> architecture is built in.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>> ---
>>   hw/virtio/virtio-mem.c | 20 ++++++++++++--------
>>   1 file changed, 12 insertions(+), 8 deletions(-)
>>
>> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
>> index 5f57eccbb66..8c40042108c 100644
>> --- a/hw/virtio/virtio-mem.c
>> +++ b/hw/virtio/virtio-mem.c
>> @@ -15,6 +15,7 @@
>>   #include "qemu/cutils.h"
>>   #include "qemu/error-report.h"
>>   #include "qemu/units.h"
>> +#include "system/arch_init.h"
>>   #include "system/numa.h"
>>   #include "system/system.h"
>>   #include "system/reset.h"
>> @@ -170,13 +171,16 @@ static bool virtio_mem_has_shared_zeropage(RAMBlock *rb)
>>    * necessary (as the section size can change). But it's more likely that the
>>    * section size will rather get smaller and not bigger over time.
>>    */
>> -#if defined(TARGET_X86_64) || defined(TARGET_I386) || defined(TARGET_S390X)
>> -#define VIRTIO_MEM_USABLE_EXTENT (2 * (128 * MiB))
>> -#elif defined(TARGET_ARM)
>> -#define VIRTIO_MEM_USABLE_EXTENT (2 * (512 * MiB))
>> -#else
>> -#error VIRTIO_MEM_USABLE_EXTENT not defined
>> -#endif
>> +static uint64_t virtio_mem_usable_extent_size(void)
>> +{
>> +    if (qemu_arch_available(QEMU_ARCH_I386 | QEMU_ARCH_S390X)) {
>> +        return 2 * 128 * MiB;
>> +    } else if (qemu_arch_available(QEMU_ARCH_ARM)) {
>> +        return 2 * 512 * MiB;
>> +    } else {
>> +        g_assert_not_reached();
>> +    }
>> +}
> 
> What happens if/when we have multiple arches available? Won't we want to
> know which CPU the virtio-mem device is attached to or do we take the
> minimal value over the whole system?

"per attached vcpu" is how I was previously considering this problem,
but IIUC from the discussions with Pierrick, we should consider single
binary as a first step before heterogeneous emulation.

If you think the minimal value is good enough, then that'd be my
preferred choice, as the simplest to implement.

> 
>>   
>>   static bool virtio_mem_is_busy(void)
>>   {
>> @@ -721,7 +725,7 @@ static void virtio_mem_resize_usable_region(VirtIOMEM *vmem,
>>                                               bool can_shrink)
>>   {
>>       uint64_t newsize = MIN(memory_region_size(&vmem->memdev->mr),
>> -                           requested_size + VIRTIO_MEM_USABLE_EXTENT);
>> +                           requested_size + virtio_mem_usable_extent_size());
>>   
>>       /* The usable region size always has to be multiples of the block size. */
>>       newsize = QEMU_ALIGN_UP(newsize, vmem->block_size);
>