Re: [PATCH] amd-iommu: Fix Guest CR3 Table following c/s 3a7947b6901

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.11.2020 15:19, Andrew Cooper wrote:
> "amd-iommu: use a bitfield for DTE" renamed iommu_dte_set_guest_cr3()'s gcr3
> parameter to gcr3_mfn but ended up with an off-by-PAGE_SIZE error when
> extracting bits from the address.
> 
> First of all, get_guest_cr3_from_dte() and iommu_dte_set_guest_cr3()
> are (almost) getters and setters for the same field, so should live together.
> 
> Rename them to dte_{get,set}_gcr3_table() to specifically avoid 'guest_cr3' in
> the name.  This field actually points to a table in memory containing an array
> of guest CR3 values.  As these functions are used for different logical
> indirections, they shouldn't use gfn/mfn terminology for their parameters.
> Switch them to use straight uint64_t full addresses.

All of this still looks to belong to "First of all ..." - did you
mean to have more in here, but forgot to actually put it in?

> --- a/xen/drivers/passthrough/amd/iommu_guest.c
> +++ b/xen/drivers/passthrough/amd/iommu_guest.c
> @@ -68,11 +68,39 @@ static void guest_iommu_disable(struct guest_iommu *iommu)
>      iommu->enabled = 0;
>  }
>  
> -static uint64_t get_guest_cr3_from_dte(struct amd_iommu_dte *dte)
> +/*
> + * The Guest CR3 Table is a table written by the guest kernel, pointing at
> + * gCR3 values for PASID transactions to use.  The Device Table Entry points
> + * at a system physical address.
> + *
> + * However, these helpers deliberately use untyped parameters without
> + * reference to gfn/mfn because they are used both for programming the real
> + * IOMMU, and interpreting a guests programming of its vIOMMU.
> + */
> +static uint64_t dte_get_gcr3_table(const struct amd_iommu_dte *dte)
>  {
>      return (((uint64_t)dte->gcr3_trp_51_31 << 31) |
>              (dte->gcr3_trp_30_15 << 15) |
> -            (dte->gcr3_trp_14_12 << 12)) >> PAGE_SHIFT;
> +            (dte->gcr3_trp_14_12 << 12));
> +}
> +
> +static void dte_set_gcr3_table(struct amd_iommu_dte *dte, uint16_t dom_id,
> +                               uint64_t addr, bool gv, uint8_t glx)
> +{
> +#define GCR3_MASK(hi, lo) (((1ul << ((hi) + 1)) - 1) & ~((1ul << (lo)) - 1))
> +
> +    /* I bit must be set when gcr3 is enabled */
> +    dte->i = true;
> +
> +    dte->gcr3_trp_14_12 = MASK_EXTR(addr, GCR3_MASK(14, 12));
> +    dte->gcr3_trp_30_15 = MASK_EXTR(addr, GCR3_MASK(30, 15));
> +    dte->gcr3_trp_51_31 = MASK_EXTR(addr, GCR3_MASK(51, 31));
> +
> +    dte->domain_id = dom_id;
> +    dte->glx = glx;
> +    dte->gv = gv;
> +
> +#undef GCR3_MASK
>  }

I realize the question is somewhat unrelated, but aren't we updating
a live DTE here? If so, are there no ordering requirements between the
writes? Might be worth putting in barrier(s) right on this occasion.

Jan