Re: [RFC KERNEL PATCH v2 2/3] xen/pvh: Unmask irq for passthrough device in PVH dom0

[Stefano Stabellini]

On Fri, 24 Nov 2023, Jiqian Chen wrote:
> This patch is to solve two problems we encountered when we try to
> passthrough a device to hvm domU base on Xen PVH dom0.
> 
> First, hvm guest will alloc a pirq and irq for a passthrough device
> by using gsi, before that, the gsi must first has a mapping in dom0,
> see Xen code pci_add_dm_done->xc_domain_irq_permission, it will call
> into Xen and check whether dom0 has the mapping. See
> XEN_DOMCTL_irq_permission->pirq_access_permitted, "current" is PVH
> dom0 and it return irq is 0, and then return -EPERM.
> This is because the passthrough device doesn't do PHYSDEVOP_map_pirq
> when thay are enabled.
> 
> Second, in PVH dom0, the gsi of a passthrough device doesn't get
> registered, but gsi must be configured for it to be able to be
> mapped into a domU.
> 
> After searching codes, we can find map_pirq and register_gsi will be
> done in function vioapic_write_redirent->vioapic_hwdom_map_gsi when
> the gsi(aka ioapic's pin) is unmasked in PVH dom0. So the problems
> can be conclude to that the gsi of a passthrough device doesn't be
> unmasked.
> 
> To solve the unmaske problem, this patch call the unmask_irq when we
> assign a device to be passthrough. So that the gsi can get registered
> and mapped in PVH dom0.


Roger, this seems to be more of a Xen issue than a Linux issue. Why do
we need the unmask check in Xen? Couldn't we just do:


diff --git a/xen/arch/x86/hvm/vioapic.c b/xen/arch/x86/hvm/vioapic.c
index 4e40d3609a..df262a4a18 100644
--- a/xen/arch/x86/hvm/vioapic.c
+++ b/xen/arch/x86/hvm/vioapic.c
@@ -287,7 +287,7 @@ static void vioapic_write_redirent(
             hvm_dpci_eoi(d, gsi);
     }
 
-    if ( is_hardware_domain(d) && unmasked )
+    if ( is_hardware_domain(d) )
     {
         /*
          * NB: don't call vioapic_hwdom_map_gsi while holding hvm.irq_lock