Re: [Qemu-devel] [PATCH v2] vfio: Add support for mmapping sub-page MMIO BARs

[Alex Williamson]

[cc +Paolo]

On Thu, 11 Aug 2016 19:05:57 +0800
Yongji Xie <address@hidden> wrote:

> Now the kernel commit 05f0c03fbac1 ("vfio-pci: Allow to mmap
> sub-page MMIO BARs if the mmio page is exclusive") allows VFIO
> to mmap sub-page BARs. This is the corresponding QEMU patch.
> With those patches applied, we could passthrough sub-page BARs
> to guest, which can help to improve IO performance for some devices.
> 
> In this patch, we expand MemoryRegions of these sub-page
> MMIO BARs to PAGE_SIZE in vfio_pci_write_config(), so that
> the BARs could be passed to KVM ioctl KVM_SET_USER_MEMORY_REGION
> with a valid size. The expanding size will be recovered when
> the base address of sub-page BAR is changed and not page aligned
> any more in guest. And we also set the priority of these BARs'
> memory regions to zero in case of overlap with BARs which share
> the same page with sub-page BARs in guest.
> 
> Signed-off-by: Yongji Xie <address@hidden>
> ---
>  hw/vfio/common.c |    3 +--
>  hw/vfio/pci.c    |   76 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 77 insertions(+), 2 deletions(-)

Hi Yongji,

There was an automated patch checker reply to this patch already, see:

https://patchwork.kernel.org/patch/9275077/

Looks like there's a trivial whitespace issue with using a tab.  Also
another QEMU style issue noted below.

> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index b313e7c..1a70307 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -661,8 +661,7 @@ int vfio_region_setup(Object *obj, VFIODevice *vbasedev, 
> VFIORegion *region,
>                                region, name, region->size);
>  
>          if (!vbasedev->no_mmap &&
> -            region->flags & VFIO_REGION_INFO_FLAG_MMAP &&
> -            !(region->size & ~qemu_real_host_page_mask)) {
> +            region->flags & VFIO_REGION_INFO_FLAG_MMAP) {
>  
>              vfio_setup_region_sparse_mmaps(region, info);
>  
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 7bfa17c..7035617 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -1057,6 +1057,65 @@ static const MemoryRegionOps vfio_vga_ops = {
>  };
>  
>  /*
> + * Expand memory region of sub-page(size < PAGE_SIZE) MMIO BAR to page
> + * size if the BAR is in an exclusive page in host so that we could map
> + * this BAR to guest. But this sub-page BAR may not occupy an exclusive
> + * page in guest. So we should set the priority of the expanded memory
> + * region to zero in case of overlap with BARs which share the same page
> + * with the sub-page BAR in guest. Besides, we should also recover the
> + * size of this sub-page BAR when its base address is changed in guest
> + * and not page aligned any more.
> + */
> +static void vfio_sub_page_bar_update_mapping(PCIDevice *pdev, int bar)
> +{
> +    VFIOPCIDevice *vdev = DO_UPCAST(VFIOPCIDevice, pdev, pdev);
> +    VFIORegion *region = &vdev->bars[bar].region;
> +    MemoryRegion *mmap_mr, *mr;
> +    PCIIORegion *r;
> +    pcibus_t bar_addr;
> +
> +    /* Make sure that the whole region is allowed to be mmapped */
> +    if (!(region->nr_mmaps == 1 &&
> +        region->mmaps[0].size == region->size)) {
> +        return;
> +    }
> +
> +    r = &pdev->io_regions[bar];
> +    bar_addr = r->addr;
> +    if (bar_addr == PCI_BAR_UNMAPPED) {
> +        return;
> +    }
> +
> +    mr = region->mem;
> +    mmap_mr = &region->mmaps[0].mem;
> +    memory_region_transaction_begin();
> +    if (memory_region_size(mr) < qemu_real_host_page_size) {
> +        if (!(bar_addr & ~qemu_real_host_page_mask) &&
> +            memory_region_is_mapped(mr) && region->mmaps[0].mmap) {
> +            /* Expand memory region to page size and set priority */
> +            memory_region_del_subregion(r->address_space, mr);
> +            memory_region_set_size(mr, qemu_real_host_page_size);
> +            memory_region_set_size(mmap_mr, qemu_real_host_page_size);
> +            memory_region_add_subregion_overlap(r->address_space,
> +                                                bar_addr, mr, 0);
> +     }
> +    } else {
> +        /* This case would happen when guest rescan one PCI device */
> +        if (bar_addr & ~qemu_real_host_page_mask) {
> +            /* Recover the size of memory region */
> +            memory_region_set_size(mr, r->size);
> +            memory_region_set_size(mmap_mr, r->size);
> +        } else if (memory_region_is_mapped(mr)) {
> +            /* Set the priority of memory region to zero */
> +            memory_region_del_subregion(r->address_space, mr);
> +            memory_region_add_subregion_overlap(r->address_space,
> +                                                bar_addr, mr, 0);
> +        }
> +    }
> +    memory_region_transaction_commit();

Paolo, as the reigning memory API expert, do you see any issues with
this?  Expanding the size of a container MemoryRegion and the contained
mmap'd subregion and manipulating priorities so that we don't interfere
with other MemoryRegions.

> +}
> +
> +/*
>   * PCI config space
>   */
>  uint32_t vfio_pci_read_config(PCIDevice *pdev, uint32_t addr, int len)
> @@ -1139,6 +1198,23 @@ void vfio_pci_write_config(PCIDevice *pdev,
>          } else if (was_enabled && !is_enabled) {
>              vfio_msix_disable(vdev);
>          }
> +    } else if (ranges_overlap(addr, len, PCI_BASE_ADDRESS_0, 24) ||
> +        range_covers_byte(addr, len, PCI_COMMAND)) {
> +        pcibus_t old_addr[PCI_NUM_REGIONS - 1];
> +        int bar;
> +
> +        for (bar = 0; bar < PCI_ROM_SLOT; bar++) {
> +            old_addr[bar] = pdev->io_regions[bar].addr;
> +        }
> +
> +        pci_default_write_config(pdev, addr, val, len);
> +
> +        for (bar = 0; bar < PCI_ROM_SLOT; bar++) {
> +            if (old_addr[bar] != pdev->io_regions[bar].addr &&
> +                pdev->io_regions[bar].size > 0 &&
> +                pdev->io_regions[bar].size < qemu_real_host_page_size)

This requires {} per QEMU coding standards.

> +                vfio_sub_page_bar_update_mapping(pdev, bar);
> +        }
>      } else {
>          /* Write everything to QEMU to keep emulated bits correct */
>          pci_default_write_config(pdev, addr, val, len);

Thanks,
Alex