Re: [PATCH v5 08/10] xen/arm: Setup MMIO range trap handlers for hardware domain

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Oct 25, 2021 at 09:38:00AM +0000, Oleksandr Andrushchenko wrote:
> Hi, Roger!
> 
> On 13.10.21 13:11, Roger Pau Monné wrote:
> > On Fri, Oct 08, 2021 at 08:55:33AM +0300, Oleksandr Andrushchenko wrote:
> >> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> >>
> >> In order for vPCI to work it needs to maintain guest and hardware
> >> domain's views of the configuration space. For example, BARs and
> >> COMMAND registers require emulation for guests and the guest view
> >> of the registers needs to be in sync with the real contents of the
> >> relevant registers. For that ECAM address space needs to also be
> >> trapped for the hardware domain, so we need to implement PCI host
> >> bridge specific callbacks to properly setup MMIO handlers for those
> >> ranges depending on particular host bridge implementation.
> >>
> >> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> >> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
> >> Reviewed-by: Rahul Singh <rahul.singh@arm.com>
> >> Tested-by: Rahul Singh <rahul.singh@arm.com>
> >> ---
> >> Since v3:
> >> - fixed comment formatting
> >> Since v2:
> >> - removed unneeded assignment (count = 0)
> >> - removed unneeded header inclusion
> >> - update commit message
> >> Since v1:
> >>   - Dynamically calculate the number of MMIO handlers required for vPCI
> >>     and update the total number accordingly
> >>   - s/clb/cb
> >>   - Do not introduce a new callback for MMIO handler setup
> >> ---
> >>   xen/arch/arm/domain.c              |  2 ++
> >>   xen/arch/arm/pci/pci-host-common.c | 28 ++++++++++++++++++++++++
> >>   xen/arch/arm/vpci.c                | 34 ++++++++++++++++++++++++++++++
> >>   xen/arch/arm/vpci.h                |  6 ++++++
> >>   xen/include/asm-arm/pci.h          |  5 +++++
> >>   5 files changed, 75 insertions(+)
> >>
> >> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> >> index 79012bf77757..fa6fcc5e467c 100644
> >> --- a/xen/arch/arm/domain.c
> >> +++ b/xen/arch/arm/domain.c
> >> @@ -733,6 +733,8 @@ int arch_domain_create(struct domain *d,
> >>       if ( (rc = domain_vgic_register(d, &count)) != 0 )
> >>           goto fail;
> >>   
> >> +    count += domain_vpci_get_num_mmio_handlers(d);
> >> +
> >>       if ( (rc = domain_io_init(d, count + MAX_IO_HANDLER)) != 0 )
> > IMO it might be better to convert the fixed array into a linked list,
> > I guess this made sense when Arm had a very limited number of mmio
> > trap handlers, but having to do all this accounting seems quite
> > tedious every time you want to add new handlers.
> Yes, I think we need to do so, but this improvement was not meant
> to be in this patch.

Ack, just wanted to raise that this model seems to be getting more
complex than just setting up a list.

> >> diff --git a/xen/arch/arm/vpci.c b/xen/arch/arm/vpci.c
> >> index 76c12b92814f..6e179cd3010b 100644
> >> --- a/xen/arch/arm/vpci.c
> >> +++ b/xen/arch/arm/vpci.c
> >> @@ -80,17 +80,51 @@ static const struct mmio_handler_ops vpci_mmio_handler = {
> >>       .write = vpci_mmio_write,
> >>   };
> >>   
> >> +static int vpci_setup_mmio_handler(struct domain *d,
> >> +                                   struct pci_host_bridge *bridge)
> >> +{
> >> +    struct pci_config_window *cfg = bridge->cfg;
> >> +
> >> +    register_mmio_handler(d, &vpci_mmio_handler,
> >> +                          cfg->phys_addr, cfg->size, NULL);
> > I'm confused here, don't you need to use a slightly different handler
> > for dom0 so that you can differentiate between the segments of the
> > host bridges?
> >
> > AFAICT the translation done by vpci_mmio_handler using MMCFG_BDF
> > always assume segment 0.
> You are absolutely right here: I can set up hwdom specific
> handlers, so I can properly translate the segment.
> On the other hand, when virtual bus topology added, the SBDF
> translation from virtual to physical SBDF resides in the Arm's
> vpci_mmio_{read|write}, like the below:
>      if ( priv->is_virt_ecam &&
>           !vpci_translate_virtual_device(v->domain, &sbdf) )
>              return 1;
> (BTW Jan asked in some other comment why it is Arm specific:
> I tend to keep it Arm specific until the point when x86 wants that
> as well. Until that point the code, if moved to common, will be
> unneeded and as Jan calls that "dead")
> So, I think that I can extend vpci_mmio_{read|write} to account
> on the hwdom like (virtual bus code is the future code):
> 
> static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
>                            register_t *r, void *p)
> {
> ...
>      struct vpci_mmio_priv *priv = (struct vpci_mmio_priv *)p;
> 
>      if ( priv->is_virt_ecam )
>          sbdf.sbdf = MMCFG_BDF(info->gpa); /* For virtual bus topology the segment is always 0. */
>      else
>      {
>          sbdf.sbdf = MMCFG_BDF(info->gpa);
>          sbdf.seg = priv->segment;
>      }
>      reg = REGISTER_OFFSET(info->gpa);
> 
> ...
>      /*
>       * For the passed through devices we need to map their virtual SBDF
>       * to the physical PCI device being passed through.
>       */
>      if ( priv->is_virt_ecam &&
>           !vpci_translate_virtual_device(v->domain, &sbdf) )
>              return 1;
> 
> Will it work for you?

Right, I guess it could work as long as the differences between the
hardware domain and the unprivileged ones are not too big.

The nice part about having different handlers is that you avoid a
bunch of conditionals (ie: no need to check for is_virt_ecam) which
makes the code easier to follow. OTOH it could introduce more code
duplication.

Thanks, Roger.