Re: [Xen-devel] [PATCH] x86/hvmloader: align BAR position to 4K

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.01.2020 19:13, Roger Pau Monne wrote:
> When placing BARs with sizes smaller than 4K multiple BARs can end
> up mapped to the same guest physical address, and thus won't work
> correctly.

BARs of the same device can very well share a page in the common
case, can't they? (There may be reasons, like things getting too
complicated, for not actually honoring this, but then the
description should say so imo.)

> Align all BARs placement to 4K in hvmloader to prevent such
> overlapping.
> 
> Note that the guest can still move the BARs around and create this
> collisions, and that BARs not filling up a physical page might leak
> access to other MMIO regions placed in the same host physical page.

Throughout the description and in the title I think you would
better say "memory BAR".

> --- a/tools/firmware/hvmloader/pci.c
> +++ b/tools/firmware/hvmloader/pci.c
> @@ -489,6 +489,10 @@ void pci_setup(void)
>  
>          resource->base = base;
>  
> +        if ( (bar_data & PCI_BASE_ADDRESS_SPACE) ==
> +             PCI_BASE_ADDRESS_SPACE_MEMORY )
> +            resource->base = ROUNDUP(resource->base, PAGE_SIZE);

Doesn't this need adjustments to the calculation of the MMIO
hole size higher up in the function?

Also, as per a few lines up, perhaps

        if ( resource == &mem_resource)

?

Jan

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