Re: [PATCH v4] hw/arm/virt: Support larger highmem MMIO regions

[Eric Auger <eric.auger@redhat.com>]

On 2/12/25 3:54 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>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> ---
> v4: - Added default size to highmem-mmio-size description
> v3: - Updated highmem-mmio-size description
> 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(+)
>
> diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
> index e67e7f0f7c50..6ff1de1ecbba 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 or equal to the default size (512G).
> +
>  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;
> +    }
> +
> +    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",
Reviewed-by: Eric Auger <eric.auger@redhat.com>

The only nitpick I have is that if you read

static MemMapEntry extended_memmap[] = {
    /* Additional 64 MB redist region (can contain up to 512
redistributors) */
    [VIRT_HIGH_GIC_REDIST2] =   { 0x0, 64 * MiB },
    [VIRT_HIGH_PCIE_ECAM] =     { 0x0, 256 * MiB },
    /* Second PCIe window */
    [VIRT_HIGH_PCIE_MMIO] =     { 0x0, 512 * GiB },
};
and the above comment, it is not obvious that the HIGH_PCI_MMIO can be
extended by an option. A distracted reader may not get it.

But I don't know if it is worth respinning.

Eric