Re: [PATCH v2] VT-d: Don't assume register-based invalidation is always supported

[Jan Beulich <jbeulich@suse.com>]

On 01.04.2020 22:06, Chao Gao wrote:
> According to Intel VT-d SPEC rev3.3 Section 6.5, Register-based Invalidation
> isn't supported by Intel VT-d version 6 and beyond.
> 
> This hardware change impacts following two scenarios: admin can disable
> queued invalidation via 'qinval' cmdline and use register-based interface;
> VT-d switches to register-based invalidation when queued invalidation needs
> to be disabled, for example, during disabling x2apic or during system
> suspension or after enabling queued invalidation fails.
> 
> To deal with this hardware change, if register-based invalidation isn't
> supported, queued invalidation cannot be disabled through Xen cmdline; and
> if queued invalidation has to be disabled temporarily in some scenarios,
> VT-d won't switch to register-based interface but use some dummy functions
> to catch errors in case there is any invalidation request issued when queued
> invalidation is disabled.
> 
> Signed-off-by: Chao Gao <chao.gao@intel.com>

In principle (with a minor nit further down)
Reviewed-by: Jan Beulich <jbeulich@suse.com>

However, ...

> ---
> Changes in v2:
>  - verify system suspension and resumption with this patch applied
>  - don't fall back to register-based interface if enabling qinval failed.
>    see the change in init_vtd_hw().
>  - remove unnecessary "queued_inval_supported" variable
>  - constify the "struct vtd_iommu *" of has_register_based_invalidation()
>  - coding-style changes

... while this suggests this is v2 of a recently sent patch, the
submission is dated a little over a year ago. This is confusing.
It is additionally confusing that there were two copies of it in
my inbox, despite mails coming from a list normally getting
de-duplicated somewhere at our end (I believe).

> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -1193,6 +1193,14 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
>  
>      iommu->cap = dmar_readq(iommu->reg, DMAR_CAP_REG);
>      iommu->ecap = dmar_readq(iommu->reg, DMAR_ECAP_REG);
> +    iommu->version = dmar_readl(iommu->reg, DMAR_VER_REG);
> +
> +    if ( !iommu_qinval && !has_register_based_invalidation(iommu) )
> +    {
> +        printk(XENLOG_WARNING VTDPREFIX "IOMMU %d: cannot disable Queued Invalidation.\n",
> +               iommu->index);

Here (and at least once more yet further down): We don't normally end
log messages with a full stop. Easily addressable while committing, of
course.

Jan