Re: [Xen-devel] [PATCH for-4.12] passthrough/vtd: Drop the "workaround_bios_bug" logic entirely

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 21, 2019 at 08:26:20PM +0000, Andrew Cooper wrote:
> It turns out that this code was previously dead.
> 
> c/s dcf41790 " x86/mmcfg/drhd: Move acpi_mmcfg_init() call before calling
> acpi_parse_dmar()" resulted in PCI segment 0 now having been initialised
> enough for acpi_parse_one_drhd() to not take the
> 
>   /* Skip checking if segment is not accessible yet. */
> 
> path unconditionally.  However, some systems have DMAR tables which list
> devices which are disabled by user choice (in particular, Dell PowerEdge R740
> with I/O AT DMA disabled), and turning off all IOMMU functionality in this
> case is entirely unhelpful behaviour.
> 
> Leave the warning which identifies the problematic devices, but drop the
> remaining logic.  This leaves the system in better overall state, and working
> in the same way that it did in previous releases.
> 
> Reported-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I think this is a more sane behavior.

> -        if ( invalid_cnt )
> -        {
> -            if ( iommu_workaround_bios_bug &&
> -                 invalid_cnt == dmaru->scope.devices_cnt )
> -            {
> -                printk(XENLOG_WARNING VTDPREFIX
> -                       "  Workaround BIOS bug: ignoring DRHD (no devices in 
> its scope are PCI discoverable)\n");
> -
> -                scope_devices_free(&dmaru->scope);
> -                iommu_free(dmaru);
> -                xfree(dmaru);
> -            }
> -            else
> -            {
> -                printk(XENLOG_WARNING VTDPREFIX
> -                       "  DRHD is invalid (some devices in its scope are not 
> PCI discoverable)\n");
> -                printk(XENLOG_WARNING VTDPREFIX
> -                       "  Try \"iommu=force\" or 
> \"iommu=workaround_bios_bug\" if you really want VT-d\n");
> -                ret = -EINVAL;
> -            }

The workaround_bios_bug option seems quite pointless anyway, it only
prevents propagating the error to the caller if all the devices in the
DMAR scope are non-existent, or else the DMAR won't be registered
and an error would be returned to the caller.

Thanks, Roger.

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