Re: [PATCH v3 2/3] amd_iommu: Turn on XT support only when guest has enabled it

[Vasant Hegde <vasant.hegde@amd.com>]

On 3/2/2026 5:21 PM, Sairaj Kodilkar wrote:
> Current code uses 32 bit destination ID irrespective of the fact that
> guest has enabled x2APIC support through control register[XTEn] and
> completely depends on command line parameter xtsup=on. This is not a
> correct hardware behaviour and can cause problems in the guest which has
> not enabled XT mode.
> 
> Introduce new flag "xten", which is enabled when guest writes 1 to the
> control register bit 50 (XTEn). Also, add a new subsection in
> `VMStateDescription` for backward compatibility during vm migration.
> 
> Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
> Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
> Reviewed-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
> ---
>  hw/i386/amd_iommu.c | 21 +++++++++++++++++++--
>  hw/i386/amd_iommu.h |  4 +++-
>  2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
> index f5aa9c763d02..4a86e62a3b92 100644
> --- a/hw/i386/amd_iommu.c
> +++ b/hw/i386/amd_iommu.c
> @@ -1523,6 +1523,8 @@ static void amdvi_handle_control_write(AMDVIState *s)
>      s->cmdbuf_enabled = s->enabled && !!(control &
>                          AMDVI_MMIO_CONTROL_CMDBUFLEN);
>      s->ga_enabled = !!(control & AMDVI_MMIO_CONTROL_GAEN);
> +    s->xten = !!(control & AMDVI_MMIO_CONTROL_XTEN) && s->xtsup &&
> +              s->ga_enabled;
>  
>      /* update the flags depending on the control register */
>      if (s->cmdbuf_enabled) {
> @@ -1996,7 +1998,7 @@ static int amdvi_int_remap_ga(AMDVIState *iommu,
>      irq->vector = irte.hi.fields.vector;
>      irq->dest_mode = irte.lo.fields_remap.dm;
>      irq->redir_hint = irte.lo.fields_remap.rq_eoi;
> -    if (iommu->xtsup) {
> +    if (iommu->xten) {
>          irq->dest = irte.lo.fields_remap.destination |
>                      (irte.hi.fields.destination_hi << 24);
>      } else {
> @@ -2379,6 +2381,7 @@ static void amdvi_init(AMDVIState *s)
>      s->mmio_enabled = false;
>      s->enabled = false;
>      s->cmdbuf_enabled = false;
> +    s->xten = false;
>  
>      /* reset MMIO */
>      memset(s->mmior, 0, AMDVI_MMIO_SIZE);
> @@ -2443,6 +2446,16 @@ static void amdvi_sysbus_reset(DeviceState *dev)
>      amdvi_reset_address_translation_all(s);
>  }
>  
> +static const VMStateDescription vmstate_xt = {
> +       .name = "amd-iommu-xt",
> +       .version_id = 1,
> +       .minimum_version_id = 1,

Wrong indentation. Otherwise patch looks good.

-Vasant