Re: [PATCH 2/4] intel_iommu: Support IR-only mode without DMA translation

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 14, 2022 at 02:25:42PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> By setting none of the SAGAW bits we can indicate to a guest that DMA
> translation isn't supported. Tested by booting Windows 10, as well as
> Linux guests with the fix at https://git.kernel.org/torvalds/c/c40aaaac10
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Acked-by: Jason Wang <jasowang@redhat.com>

this is borderline like a feature, but ...

> ---
>  hw/i386/intel_iommu.c         | 14 ++++++++++----
>  include/hw/i386/intel_iommu.h |  1 +
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 32471a44cb..948c653e74 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -2214,7 +2214,7 @@ static void vtd_handle_gcmd_write(IntelIOMMUState *s)
>      uint32_t changed = status ^ val;
>  
>      trace_vtd_reg_write_gcmd(status, val);
> -    if (changed & VTD_GCMD_TE) {
> +    if ((changed & VTD_GCMD_TE) && s->dma_translation) {
>          /* Translation enable/disable */
>          vtd_handle_gcmd_te(s, val & VTD_GCMD_TE);
>      }
> @@ -3122,6 +3122,7 @@ static Property vtd_properties[] = {
>      DEFINE_PROP_BOOL("x-scalable-mode", IntelIOMMUState, scalable_mode, FALSE),
>      DEFINE_PROP_BOOL("snoop-control", IntelIOMMUState, snoop_control, false),
>      DEFINE_PROP_BOOL("dma-drain", IntelIOMMUState, dma_drain, true),
> +    DEFINE_PROP_BOOL("dma-translation", IntelIOMMUState, dma_translation, true),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> @@ -3627,12 +3628,17 @@ static void vtd_init(IntelIOMMUState *s)
>      s->next_frcd_reg = 0;
>      s->cap = VTD_CAP_FRO | VTD_CAP_NFR | VTD_CAP_ND |
>               VTD_CAP_MAMV | VTD_CAP_PSI | VTD_CAP_SLLPS |
> -             VTD_CAP_SAGAW_39bit | VTD_CAP_MGAW(s->aw_bits);
> +             VTD_CAP_MGAW(s->aw_bits);
>      if (s->dma_drain) {
>          s->cap |= VTD_CAP_DRAIN;
>      }
> -    if (s->aw_bits == VTD_HOST_AW_48BIT) {
> -        s->cap |= VTD_CAP_SAGAW_48bit;
> +    if (s->dma_translation) {
> +            if (s->aw_bits >= VTD_HOST_AW_39BIT) {
> +                    s->cap |= VTD_CAP_SAGAW_39bit;
> +            }
> +            if (s->aw_bits >= VTD_HOST_AW_48BIT) {
> +                    s->cap |= VTD_CAP_SAGAW_48bit;
> +            }
>      }
>      s->ecap = VTD_ECAP_QI | VTD_ECAP_IRO;
>


... this looks like you are actually fixing aw_bits < VTD_HOST_AW_39BIT,
right? So maybe this patch is ok like this since it also fixes a
bug. Pls add this to commit log though.

  
> diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h
> index 3b5ac869db..d898be85ce 100644
> --- a/include/hw/i386/intel_iommu.h
> +++ b/include/hw/i386/intel_iommu.h
> @@ -267,6 +267,7 @@ struct IntelIOMMUState {
>      bool buggy_eim;                 /* Force buggy EIM unless eim=off */
>      uint8_t aw_bits;                /* Host/IOVA address width (in bits) */
>      bool dma_drain;                 /* Whether DMA r/w draining enabled */
> +    bool dma_translation;           /* Whether DMA translation supported */
>  
>      /*
>       * Protects IOMMU states in general.  Currently it protects the
> -- 
> 2.33.1