Re: [QEMU PATCH v8] xen/passthrough: use gsi to map pirq when dom0 is PVH

[Stewart Hildebrand <stewart.hildebrand@amd.com>]

On 10/16/24 02:28, Jiqian Chen wrote:
> In PVH dom0, when passthrough a device to domU, QEMU code
> xen_pt_realize->xc_physdev_map_pirq wants to use gsi, but in current codes
> the gsi number is got from file /sys/bus/pci/devices/<sbdf>/irq, that is
> wrong, because irq is not equal with gsi, they are in different spaces, so
> pirq mapping fails.
> 
> To solve above problem, use new interface of Xen, xc_pcidev_get_gsi to get
> gsi and use xc_physdev_map_pirq_gsi to map pirq when dom0 is PVH.
> 
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> Signed-off-by: Huang Rui <ray.huang@amd.com>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> ---
> Hi All,
> This is v8 to support passthrough on Xen when dom0 is PVH.
> v7->v8 change:
> * Since xc_physdev_gsi_from_dev was renamed to xc_pcidev_get_gsi, changed it.
> * Added xen_run_qemu_on_hvm to check if Qemu run on PV dom0, if not use xc_physdev_map_pirq_gsi to map pirq.
> * Used CONFIG_XEN_CTRL_INTERFACE_VERSION to wrap the new part for compatibility.
> * Added "#define DOMID_RUN_QEMU 0" to represent the id of domain that Qemu run on.
> 
> 
> Best regards,
> Jiqian Chen
> 
> 
> 
> v6->v7 changes:
> * Because the function of obtaining gsi was changed on the kernel and Xen side. Changed to use
>   xc_physdev_gsi_from_dev, that requires passing in sbdf instead of irq.
> 
> v5->v6 changes:
> * Because the function of obtaining gsi was changed on the kernel and Xen side. Changed to use
>   xc_physdev_gsi_from_irq, instead of gsi sysfs.
> * Since function changed, removed the Review-by of Stefano.
> 
> v4->v5 changes:
> * Added Review-by Stefano.
> 
> v3->v4 changes:
> * Added gsi into struct XenHostPCIDevice and used gsi number that read from gsi sysfs
>   if it exists, if there is no gsi sysfs, still use irq.
> 
> v2->v3 changes:
> * Due to changes in the implementation of the second patch on kernel side(that adds
>   a new sysfs for gsi instead of a new syscall), so read gsi number from the sysfs of gsi.
> 
> v1 and v2:
> We can record the relation between gsi and irq, then when userspace(qemu) want
> to use gsi, we can do a translation. The third patch of kernel(xen/privcmd: Add new syscall
> to get gsi from irq) records all the relations in acpi_register_gsi_xen_pvh() when dom0
> initialize pci devices, and provide a syscall for userspace to get the gsi from irq. The
> third patch of xen(tools: Add new function to get gsi from irq) add a new function
> xc_physdev_gsi_from_irq() to call the new syscall added on kernel side.
> And then userspace can use that function to get gsi. Then xc_physdev_map_pirq() will success.
> 
> Issues we encountered:
> 1. failed to map pirq for gsi
> Problem: qemu will call xc_physdev_map_pirq() to map a passthrough device's gsi to pirq in
> function xen_pt_realize(). But failed.
> 
> Reason: According to the implement of xc_physdev_map_pirq(), it needs gsi instead of irq,
> but qemu pass irq to it and treat irq as gsi, it is got from file
> /sys/bus/pci/devices/xxxx:xx:xx.x/irq in function xen_host_pci_device_get(). But actually
> the gsi number is not equal with irq. They are in different space.
> ---
>  hw/xen/xen_pt.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  hw/xen/xen_pt.h |  1 +
>  2 files changed, 45 insertions(+)
> 
> diff --git a/hw/xen/xen_pt.c b/hw/xen/xen_pt.c
> index 3635d1b39f79..7f8139d20915 100644
> --- a/hw/xen/xen_pt.c
> +++ b/hw/xen/xen_pt.c
> @@ -766,6 +766,41 @@ static void xen_pt_destroy(PCIDevice *d) {
>  }
>  /* init */
>  
> +#define PCI_SBDF(seg, bus, dev, func) \
> +            ((((uint32_t)(seg)) << 16) | \
> +            (PCI_BUILD_BDF(bus, PCI_DEVFN(dev, func))))

Nit: This macro looks generic and useful. Would it be better defined in
include/hw/pci/pci.h?

> +
> +#if CONFIG_XEN_CTRL_INTERFACE_VERSION >= 42000
> +static bool xen_run_qemu_on_hvm(void)

This function name seems to imply "is qemu running on HVM?", but I think
the question we're really trying to answer is whether the pcidev needs
a GSI mapped. How about calling the function "xen_pt_needs_gsi" or
similar?

> +{
> +    xc_domaininfo_t info;
> +
> +    if (!xc_domain_getinfo_single(xen_xc, DOMID_RUN_QEMU, &info) &&
> +        (info.flags & XEN_DOMINF_hvm_guest)) {

I think reading /sys/hypervisor/guest_type would allow you to get the
same information without another hypercall.

> +        return true;
> +    }
> +
> +    return false;
> +}
> +
> +static int xen_map_pirq_for_gsi(PCIDevice *d, int *pirq)

Nit: s/xen_/xen_pt_/

> +{
> +    int gsi;
> +    XenPCIPassthroughState *s = XEN_PT_DEVICE(d);
> +
> +    gsi = xc_pcidev_get_gsi(xen_xc,
> +                            PCI_SBDF(s->real_device.domain,
> +                                     s->real_device.bus,
> +                                     s->real_device.dev,
> +                                     s->real_device.func));
> +    if (gsi >= 0) {
> +        return xc_physdev_map_pirq_gsi(xen_xc, xen_domid, gsi, pirq);
> +    }
> +
> +    return gsi;
> +}
> +#endif
> +
>  static void xen_pt_realize(PCIDevice *d, Error **errp)
>  {
>      ERRP_GUARD();
> @@ -847,7 +882,16 @@ static void xen_pt_realize(PCIDevice *d, Error **errp)
>          goto out;
>      }
>  
> +#if CONFIG_XEN_CTRL_INTERFACE_VERSION >= 42000
> +    if (xen_run_qemu_on_hvm()) {
> +        rc = xen_map_pirq_for_gsi(d, &pirq);
> +    } else {
> +        rc = xc_physdev_map_pirq(xen_xc, xen_domid, machine_irq, &pirq);
> +    }
> +#else
>      rc = xc_physdev_map_pirq(xen_xc, xen_domid, machine_irq, &pirq);
> +#endif
> +
>      if (rc < 0) {
>          XEN_PT_ERR(d, "Mapping machine irq %u to pirq %i failed, (err: %d)\n",
>                     machine_irq, pirq, errno);
> diff --git a/hw/xen/xen_pt.h b/hw/xen/xen_pt.h
> index 095a0f0365d4..a08b45b7fbae 100644
> --- a/hw/xen/xen_pt.h
> +++ b/hw/xen/xen_pt.h
> @@ -36,6 +36,7 @@ void xen_pt_log(const PCIDevice *d, const char *f, ...) G_GNUC_PRINTF(2, 3);
>  #  define XEN_PT_LOG_CONFIG(d, addr, val, len)
>  #endif
>  
> +#define DOMID_RUN_QEMU 0
>  
>  /* Helper */
>  #define XEN_PFN(x) ((x) >> XC_PAGE_SHIFT)