RE: [PATCH V2 02/23] xen/ioreq: Make x86's IOREQ feature common

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Oleksandr <olekstysh@gmail.com>
> Sent: 17 November 2020 14:48
> To: paul@xen.org
> Cc: xen-devel@lists.xenproject.org; 'Oleksandr Tyshchenko' <oleksandr_tyshchenko@epam.com>; 'Andrew
> Cooper' <andrew.cooper3@citrix.com>; 'George Dunlap' <george.dunlap@citrix.com>; 'Ian Jackson'
> <iwj@xenproject.org>; 'Jan Beulich' <jbeulich@suse.com>; 'Julien Grall' <julien@xen.org>; 'Stefano
> Stabellini' <sstabellini@kernel.org>; 'Wei Liu' <wl@xen.org>; 'Roger Pau Monné'
> <roger.pau@citrix.com>; 'Tim Deegan' <tim@xen.org>; 'Julien Grall' <julien.grall@arm.com>
> Subject: Re: [PATCH V2 02/23] xen/ioreq: Make x86's IOREQ feature common
> 
> 
> Hi Paul
> 

Hi Oleksandr,

> >
> >> The 'legacy' mechanism of mapping magic pages for ioreq servers
> >> should remain x86 specific I think that aspect of the code needs to
> >> remain behind and not get moved into common code. You could do that
> >> in arch specific calls in hvm_ioreq_server_enable/disable() and
> >> hvm_get_ioreq_server_info().
> > Well, if legacy mechanism is not going to be used for other arch and
> > should remain x86 specific, I will try to investigate what should be
> > left in x86 code and rework the series.
> > As a side note, I am afraid, we won't get a 100% code movement (which
> > I managed to achieve here) for the next version of this patch as we
> > need arch/x86/hvm/ioreq.c.
> 
> I am investigating how to split the code in order to leave the 'legacy'
> mechanism x86 specific and have a few questions. Could you please
> clarify the following:
> 
> 1. The split of hvm_ioreq_server_enable/disable() is obvious to me, I
> would like to clarify regarding hvm_get_ioreq_server_info().
> Is it close to what you had in mind when suggesting the split of
> hvm_get_ioreq_server_info() or I just need to abstract
> hvm_ioreq_server_map_pages() call only?

I think it is sufficient to just abstract hvm_ioreq_server_map_pages() (and return -EOPNOTSUPP in the Arm case).
The buf ioreq port should be common.

> (Not completed and non tested)
> 
> +/* Called with ioreq_server lock held */
> +int arch_ioreq_server_get_info(struct hvm_ioreq_server *s,
> +                               unsigned long *ioreq_gfn,
> +                               unsigned long *bufioreq_gfn,
> +                               evtchn_port_t *bufioreq_port)
> +{
> +    if ( ioreq_gfn || bufioreq_gfn )
> +    {
> +        int rc = hvm_ioreq_server_map_pages(s);
> +
> +        if ( rc )
> +            return rc;
> +    }
> +
> +    if ( ioreq_gfn )
> +        *ioreq_gfn = gfn_x(s->ioreq.gfn);
> +
> +    if ( HANDLE_BUFIOREQ(s) )
> +    {
> +        if ( bufioreq_gfn )
> +            *bufioreq_gfn = gfn_x(s->bufioreq.gfn);
> +
> +        if ( bufioreq_port )
> +            *bufioreq_port = s->bufioreq_evtchn;
> +    }
> +
> +    return 0;
> +}
> +
>   int hvm_get_ioreq_server_info(struct domain *d, ioservid_t id,
>                                 unsigned long *ioreq_gfn,
>                                 unsigned long *bufioreq_gfn,
> @@ -916,26 +954,7 @@ int hvm_get_ioreq_server_info(struct domain *d,
> ioservid_t id,
>       if ( s->emulator != current->domain )
>           goto out;
> 
> -    if ( ioreq_gfn || bufioreq_gfn )
> -    {
> -        rc = hvm_ioreq_server_map_pages(s);
> -        if ( rc )
> -            goto out;
> -    }
> -
> -    if ( ioreq_gfn )
> -        *ioreq_gfn = gfn_x(s->ioreq.gfn);
> -
> -    if ( HANDLE_BUFIOREQ(s) )
> -    {
> -        if ( bufioreq_gfn )
> -            *bufioreq_gfn = gfn_x(s->bufioreq.gfn);
> -
> -        if ( bufioreq_port )
> -            *bufioreq_port = s->bufioreq_evtchn;
> -    }
> -
> -    rc = 0;
> +    rc = arch_ioreq_server_get_info(s, ioreq_gfn, bufioreq_gfn,
> bufioreq_port);
> 
>    out:
>       spin_unlock_recursive(&d->arch.hvm.ioreq_server.lock);
> 
> 2. If I understand the code correctly, besides of the above-mentioned
> functions the arch specific calls should be in hvm_ioreq_server_init()
> and hvm_ioreq_server_deinit() to actually hide
> "hvm_ioreq_server_unmap_pages" usage from the common code.  I noticed
> that the rollback code in hvm_ioreq_server_init() and the whole
> hvm_ioreq_server_deinit() have a lot in common except an extra ASSERT()
> and hvm_ioreq_server_free_pages() call in the latter. My question is
> could we just replace the rollback code by hvm_ioreq_server_deinit()? I
> assume an extra hvm_ioreq_server_free_pages() call wouldn't be an issue
> here, but I am not sure what to do with the ASSERT, I expect it to be
> triggered at such an early stage (so it probably needs moving out of the
> hvm_ioreq_server_deinit() (or dropped?) as well as comment needs
> updating). I am asking, because this way we would get *a single* arch
> hook here which would be arch_ioreq_server_deinit() and remove code
> duplication a bit.

I would arch specific init and deinit, even if one of them does nothing... but then I like symmetry :-)

> 
> Something close to this.
> (Not completed and non tested)
> 
> @@ -761,18 +771,17 @@ static int hvm_ioreq_server_init(struct
> hvm_ioreq_server *s,
>       return 0;
> 
>    fail_add:
> -    hvm_ioreq_server_remove_all_vcpus(s);
> -    hvm_ioreq_server_unmap_pages(s);
> -
> -    hvm_ioreq_server_free_rangesets(s);
> -
> -    put_domain(s->emulator);
> +    hvm_ioreq_server_deinit(s);
>       return rc;
>   }
> 
> +void arch_ioreq_server_deinit(struct hvm_ioreq_server *s)
> +{
> +    hvm_ioreq_server_unmap_pages(s);
> +}
> +
>   static void hvm_ioreq_server_deinit(struct hvm_ioreq_server *s)
>   {
> -    ASSERT(!s->enabled);

I assume this is the ASSERT you're referring to... There's no way we should be deinit-ing an enabled server so that should remain in common code as is.

  Paul

>       hvm_ioreq_server_remove_all_vcpus(s);
> 
>       /*
> @@ -784,7 +793,7 @@ static void hvm_ioreq_server_deinit(struct
> hvm_ioreq_server *s)
>        *       the page_info pointer to NULL, meaning the latter will do
>        *       nothing.
>        */
> -    hvm_ioreq_server_unmap_pages(s);
> +    arch_ioreq_server_deinit(s);
>       hvm_ioreq_server_free_pages(s);
> 
>       hvm_ioreq_server_free_rangesets(s);
> 
>       put_domain(s->emulator);
> 
> 
> --
> 
> Regards,
> 
> Oleksandr Tyshchenko