Re: [PATCH v2] hw/arm/virt: Support larger highmem MMIO regions

[Gavin Shan <gshan@redhat.com>]

On 2/5/25 2:19 PM, Matthew R. Ochs wrote:
> The MMIO region size required to support virtualized environments with
> large PCI BAR regions can exceed the hardcoded limit configured in QEMU.
> For example, a VM with multiple NVIDIA Grace-Hopper GPUs passed through
> requires more MMIO memory than the amount provided by VIRT_HIGH_PCIE_MMIO
> (currently 512GB). Instead of updating VIRT_HIGH_PCIE_MMIO, introduce a
> new parameter, highmem-mmio-size, that specifies the MMIO size required
> to support the VM configuration.
> 
> Example usage with 1TB MMIO region size:
> 	-machine virt,gic-version=3,highmem-mmio-size=1T
> 
> Signed-off-by: Matthew R. Ochs <mochs@nvidia.com>
> ---
> v2: - Add unit suffix to example in commit message
>      - Use existing "high memory region" terminology
>      - Resolve minor braces nit
> 
>   docs/system/arm/virt.rst |  4 ++++
>   hw/arm/virt.c            | 38 ++++++++++++++++++++++++++++++++++++++
>   2 files changed, 42 insertions(+)
> 

With the following nitpick addressed:

Reviewed-by: Gavin Shan <gshan@redhat.com>

> diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
> index e67e7f0f7c50..f96cf4da2a78 100644
> --- a/docs/system/arm/virt.rst
> +++ b/docs/system/arm/virt.rst
> @@ -138,6 +138,10 @@ highmem-mmio
>     Set ``on``/``off`` to enable/disable the high memory region for PCI MMIO.
>     The default is ``on``.
>   
> +highmem-mmio-size
> +  Set the high memory region size for PCI MMIO. Must be a power-of-2 and
> +  greater than the default size.
> +

The description doesn't match with the code changes because it's acceptable
when the specified size is equal to the default one. I think the description
needs to be improved for this.

>   gic-version
>     Specify the version of the Generic Interrupt Controller (GIC) to provide.
>     Valid values are:
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 49eb0355ef0c..d8d62df43f04 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -2773,6 +2773,36 @@ static void virt_set_highmem_mmio(Object *obj, bool value, Error **errp)
>       vms->highmem_mmio = value;
>   }
>   
> +static void virt_get_highmem_mmio_size(Object *obj, Visitor *v, const char *name,
> +                          void *opaque, Error **errp)
> +{
> +    uint64_t size = extended_memmap[VIRT_HIGH_PCIE_MMIO].size;
> +
> +    visit_type_size(v, name, &size, errp);
> +}
> +
> +static void virt_set_highmem_mmio_size(Object *obj, Visitor *v, const char *name,
> +                          void *opaque, Error **errp)
> +{
> +    uint64_t size;
> +
> +    if (!visit_type_size(v, name, &size, errp)) {
> +        return;
> +    }
> +
> +    if (!is_power_of_2(size)) {
> +        error_setg(errp, "highmem_mmio_size is not a power-of-2");
> +        return;
> +    }
> +
> +    if (size < extended_memmap[VIRT_HIGH_PCIE_MMIO].size) {
> +        error_setg(errp, "highmem_mmio_size is less than the default (%lu)",
> +                   extended_memmap[VIRT_HIGH_PCIE_MMIO].size);
> +        return;
> +    }
> +

It's acceptable when the specified size is equal to the default one, slightly
conflicting with the description in docs/system/arm/virt.rst


> +    extended_memmap[VIRT_HIGH_PCIE_MMIO].size = size;
> +}
>   
>   static bool virt_get_its(Object *obj, Error **errp)
>   {
> @@ -3446,6 +3476,14 @@ static void virt_machine_class_init(ObjectClass *oc, void *data)
>                                             "Set on/off to enable/disable high "
>                                             "memory region for PCI MMIO");
>   
> +    object_class_property_add(oc, "highmem-mmio-size", "size",
> +                                   virt_get_highmem_mmio_size,
> +                                   virt_set_highmem_mmio_size,
> +                                   NULL, NULL);
> +    object_class_property_set_description(oc, "highmem-mmio-size",
> +                                          "Set the high memory region size "
> +                                          "for PCI MMIO");
> +
>       object_class_property_add_str(oc, "gic-version", virt_get_gic_version,
>                                     virt_set_gic_version);
>       object_class_property_set_description(oc, "gic-version",

Thanks,
Gavin