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

[Peter Maydell]

On Wed, 12 Feb 2025 at 14:55, Matthew R. Ochs <mochs@nvidia.com> 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

Nit: no hyphens in "power of 2" here, or in the error message text.

> +  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");

In these error messages we want to use the property name,
which has hyphens, not underscores.

> +        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);

The size member here is a hwaddr, so %lu is the wrong format
string for it (it won't compile on 32-bit hosts). You want
HWADDR_PRIu. Printing 512GB as a decimal integer isn't
very user friendly in any case.

> +        return;
> +    }
> +
> +    extended_memmap[VIRT_HIGH_PCIE_MMIO].size = size;

Because we set this field to the new size, if we try to
set the property twice, once to a large value and then again
to a value smaller than the first one, we'll print an
odd error message claiming the default value to be something
other than the default value. (Luckily I think you can't
do this via the command line; but you might be able to via QMP.)

Suggestion:

/* If changing this, update the docs for highmem-mmio-size */
#define DEFAULT_HIGH_PCIE_MMIO_SIZE_GB 512
#define DEFAULT_HIGH_PCIE_MMIO_SIZE (DEFAULT_HIGH_PCIE_MMIO_SIZE_GB * GiB)

and use it in the definition of extended_memmap[] and in
the "if (size < ...)" test for the "smaller than default" check.

Have the error message say
   "highmem_mmio_size cannot be set to a lower value than the default (%d GiB)",
   DEFAULT_HIGH_PCIE_MMIO_SIZE_GB

I agree also with Eric that we should update the comment
on extended_memmap[] with e.g.

 * Note that the highmem_mmio_size property will update the
 * high PCIE MMIO size field in this array.

thanks
-- PMM