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

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Date:   Wed, 29 Nov 2023 19:53:59 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Jiqian Chen <Jiqian.Chen@....com>, roger.pau@...rix.com
cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@....com>,
        Alex Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Stewart Hildebrand <Stewart.Hildebrand@....com>,
        Xenia Ragiadakou <xenia.ragiadakou@....com>,
        Honglei Huang <Honglei1.Huang@....com>,
        Julia Zhang <Julia.Zhang@....com>,
        Huang Rui <Ray.Huang@....com>, Huang Rui <ray.huang@....com>
Subject: Re: [RFC KERNEL PATCH v2 2/3] xen/pvh: Unmask irq for passthrough
 device in PVH dom0

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