Re: [PATCH v2 11/11] xen/arm: Process pending vPCI map/unmap operations

[Jan Beulich <jbeulich@suse.com>]

+ Paul (retaining full context for this reason)

On 23.09.2021 14:54, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> 
> vPCI may map and unmap PCI device memory (BARs) being passed through which
> may take a lot of time. For this those operations may be deferred to be
> performed later, so that they can be safely preempted.
> Run the corresponding vPCI code while switching a vCPU.
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> 
> ---
> Since v1:
>  - Moved the check for pending vpci work from the common IOREQ code
>    to hvm_do_resume on x86

While perhaps obvious for Arm folks, I'd like to see the reason for this
spelled out in the description.

> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -549,6 +549,12 @@ void hvm_do_resume(struct vcpu *v)
>      if ( !vcpu_ioreq_handle_completion(v) )
>          return;
>  
> +    if ( has_vpci(v->domain) && vpci_process_pending(v) )
> +    {
> +        raise_softirq(SCHEDULE_SOFTIRQ);
> +        return;
> +    }

Note that you're altering behavior here: Originally this was done ...

> @@ -212,19 +210,12 @@ static bool wait_for_io(struct ioreq_vcpu *sv, ioreq_t *p)
>  
>  bool vcpu_ioreq_handle_completion(struct vcpu *v)
>  {
> -    struct domain *d = v->domain;
>      struct vcpu_io *vio = &v->io;
>      struct ioreq_server *s;
>      struct ioreq_vcpu *sv;
>      enum vio_completion completion;
>      bool res = true;
>  
> -    if ( has_vpci(d) && vpci_process_pending(v) )
> -    {
> -        raise_softirq(SCHEDULE_SOFTIRQ);
> -        return false;
> -    }

... first thing. And I think it wants (perhaps even needs) to remain
that way; otherwise you'll need to explain why not, and why the change
is correct / safe.

Jan