Re: [PATCH V2 09/10] iommu/ipmmu-vmsa: Use refcount for the micro-TLBs

[Julien Grall <julien@xxxxxxx>]

Hi Oleksandr,

On 20/12/2021 21:15, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> Reference-count the micro-TLBs as several bus masters can be
> connected to the same micro-TLB (and drop TODO comment).
> This wasn't an issue so far, since the platform devices
> (this driver deals with) get assigned/deassigned together during
> domain creation/destruction. But, in order to support PCI devices
> (which are hot-pluggable) in the near future we will need to
> take care of.
Looking at the code, it is not possible to share the micro-TLB between 
domains (or even Xen). So technically, we will still want to {, 
un}hotplug the devices using the same u-TLB together.
Therefore, I would clarify that this is necessary because even if we 
have to remove all the devices together, the IOMMU driver will be 
de-assigning them one-by-one.
I would add a similar comment in the code as well.

> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@xxxxxxxxxxx>
> ---
> Changes V1 -> V2:
>     - add R-b
>     - add ASSERT() in ipmmu_utlb_disable()
> ---
>   xen/drivers/passthrough/arm/ipmmu-vmsa.c | 19 +++++++++++--------
>   1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/xen/drivers/passthrough/arm/ipmmu-vmsa.c 
> b/xen/drivers/passthrough/arm/ipmmu-vmsa.c
> index 649b9f6..1224ea4 100644
> --- a/xen/drivers/passthrough/arm/ipmmu-vmsa.c
> +++ b/xen/drivers/passthrough/arm/ipmmu-vmsa.c
> @@ -127,6 +127,7 @@ struct ipmmu_vmsa_device {
>       spinlock_t lock;    /* Protects ctx and domains[] */
>       DECLARE_BITMAP(ctx, IPMMU_CTX_MAX);
>       struct ipmmu_vmsa_domain *domains[IPMMU_CTX_MAX];
> +    unsigned int utlb_refcount[IPMMU_UTLB_MAX];
>       const struct ipmmu_features *features;
>   };
>   
> @@ -467,13 +468,12 @@ static int ipmmu_utlb_enable(struct ipmmu_vmsa_domain *domain,
>           }
>       }
>   
> -    /*
> -     * TODO: Reference-count the micro-TLB as several bus masters can be
> -     * connected to the same micro-TLB.
> -     */
> -    ipmmu_imuasid_write(mmu, utlb, 0);
> -    ipmmu_imuctr_write(mmu, utlb, imuctr |
> -                       IMUCTR_TTSEL_MMU(domain->context_id) | IMUCTR_MMUEN);
> +    if ( mmu->utlb_refcount[utlb]++ == 0 )
> +    {
> +        ipmmu_imuasid_write(mmu, utlb, 0);
> +        ipmmu_imuctr_write(mmu, utlb, imuctr |
> +                           IMUCTR_TTSEL_MMU(domain->context_id) | 
> IMUCTR_MMUEN);
> +    }
>   
>       return 0;
>   }
> @@ -484,7 +484,10 @@ static void ipmmu_utlb_disable(struct ipmmu_vmsa_domain 
> *domain,
>   {
>       struct ipmmu_vmsa_device *mmu = domain->mmu;
>   
> -    ipmmu_imuctr_write(mmu, utlb, 0);
> +    ASSERT(mmu->utlb_refcount[utlb] > 0);
> +
> +    if ( --mmu->utlb_refcount[utlb] == 0 )
> +        ipmmu_imuctr_write(mmu, utlb, 0);
>   }
>   
>   /* Domain/Context Management */
Cheers,

--
Julien Grall