Re: [PATCH v2 9/9] vfio/igd: add x-igd-gms option back to set DSM region size for guest

[Corvin Köhne]

On Tue, 2024-12-03 at 21:35 +0800, Tomita Moeko wrote:
> CAUTION: External Email!!
> DSM region is likely to store framebuffer in Windows, a small DSM
> region may cause display issues (e.g. half of the screen is black).
> By default, QEMU uses host's original value, which is determined by
> DVMT Pre-Allocated option in Intel FSP of host bios. Some vendors
> do not expose this config item to users. In such cases, x-igd-gms
> option can be used to manually set the data stolen memory size for
> guest.
> 
> When DVMT Pre-Allocated option is available in host BIOS, user should
> set DSM region size there instead of using x-igd-gms option.
> 

It might be worth linking the commit which has removed x-igd-gms and mention
that the behaviour changed slightly. Previously, the DSM region size was set to
0 if x-igd-gms was unset. This patch keeps the host value if x-igd-gms is unset.

> Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
> ---
>  hw/vfio/igd.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
> index e464cd6949..0814730f40 100644
> --- a/hw/vfio/igd.c
> +++ b/hw/vfio/igd.c
> @@ -717,6 +717,23 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int
> nr)
>  
>      QLIST_INSERT_HEAD(&vdev->bars[nr].quirks, quirk, next);
>  
> +    /*
> +     * Allow user to override dsm size using x-igd-gms option, in multiples
> of
> +     * 32MiB. This option should only be used when the desired size cannot be
> +     * set from DVMT Pre-Allocated option in host BIOS.
> +     */
> +    if (vdev->igd_gms) {
> +        if (gen < 8 && vdev->igd_gms <= 0x10) {

This doesn't work as intended for values larger than 0x10. For those values,
qemu ignores the generation and set GMS as it would be a gen 8 or later device.

> +            gmch &= ~(IGD_GMCH_GEN6_GMS_MASK << IGD_GMCH_GEN6_GMS_SHIFT);
> +            gmch |= vdev->igd_gms << IGD_GMCH_GEN6_GMS_SHIFT;
> +        } else if (vdev->igd_gms <= 0x40) {
> +            gmch &= ~(IGD_GMCH_GEN8_GMS_MASK << IGD_GMCH_GEN8_GMS_SHIFT);
> +            gmch |= vdev->igd_gms << IGD_GMCH_GEN8_GMS_SHIFT;
> +        } else {
> +            error_report("Unsupported IGD GMS value 0x%x", vdev->igd_gms);
> +        }
> +    }
> +
>      ggms_size = igd_gtt_memory_size(gen, gmch);
>      gms_size = igd_stolen_memory_size(gen, gmch);
>  

-- 
Kind regards,
Corvin

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