Re: [PATCH v3 05/11] vpci/header: Implement guest BAR register handlers

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 30, 2021 at 10:52:17AM +0300, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> 
> Emulate guest BAR register values: this allows creating a guest view
> of the registers and emulates size and properties probe as it is done
> during PCI device enumeration by the guest.
> 
> ROM BAR is only handled for the hardware domain and for guest domains
> there is a stub: at the moment PCI expansion ROM is x86 only, so it
> might not be used by other architectures without emulating x86. Other
> use-cases may include using that expansion ROM before Xen boots, hence
> no emulation is needed in Xen itself. Or when a guest wants to use the
> ROM code which seems to be rare.
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
> Since v1:
>  - re-work guest read/write to be much simpler and do more work on write
>    than read which is expected to be called more frequently
>  - removed one too obvious comment
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> ---
>  xen/drivers/vpci/header.c | 30 +++++++++++++++++++++++++++++-
>  xen/include/xen/vpci.h    |  3 +++
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
> index 1ce98795fcca..ec4d215f36ff 100644
> --- a/xen/drivers/vpci/header.c
> +++ b/xen/drivers/vpci/header.c
> @@ -400,12 +400,38 @@ static void bar_write(const struct pci_dev *pdev, 
> unsigned int reg,
>  static void guest_bar_write(const struct pci_dev *pdev, unsigned int reg,
>                              uint32_t val, void *data)
>  {
> +    struct vpci_bar *bar = data;
> +    bool hi = false;
> +
> +    if ( bar->type == VPCI_BAR_MEM64_HI )
> +    {
> +        ASSERT(reg > PCI_BASE_ADDRESS_0);
> +        bar--;
> +        hi = true;
> +    }
> +    else
> +    {
> +        val &= PCI_BASE_ADDRESS_MEM_MASK;
> +        val |= bar->type == VPCI_BAR_MEM32 ? PCI_BASE_ADDRESS_MEM_TYPE_32
> +                                           : PCI_BASE_ADDRESS_MEM_TYPE_64;
> +        val |= bar->prefetchable ? PCI_BASE_ADDRESS_MEM_PREFETCH : 0;
> +    }
> +
> +    bar->guest_addr &= ~(0xffffffffull << (hi ? 32 : 0));
> +    bar->guest_addr |= (uint64_t)val << (hi ? 32 : 0);
> +
> +    bar->guest_addr &= ~(bar->size - 1) | ~PCI_BASE_ADDRESS_MEM_MASK;
>  }
>  
>  static uint32_t guest_bar_read(const struct pci_dev *pdev, unsigned int reg,
>                                 void *data)
>  {
> -    return 0xffffffff;
> +    const struct vpci_bar *bar = data;
> +
> +    if ( bar->type == VPCI_BAR_MEM64_HI )
> +        return bar->guest_addr >> 32;
> +
> +    return bar->guest_addr;

I think this is missing a check for whether the BAR is the high part
of a 64bit one? Ie:

struct vpci_bar *bar = data;
bool hi = false;

if ( bar->type == VPCI_BAR_MEM64_HI )
{
    ASSERT(reg > PCI_BASE_ADDRESS_0);
    bar--;
    hi = true;
}

return bar->guest_addr >> (hi ? 32 : 0);

Or else when accessing the high part of a 64bit BAR you will always
return 0s as it hasn't been setup by guest_bar_write.

Thanks, Roger.