[PATCH] xen/pt: fix igd passthrough for pc machine with xen accelerator

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

Commit 998250e97661 ("xen, gfx passthrough: register host bridge specific
to passthrough") uses the igd-passthrough-i440FX pci host device with
the xenfv machine type and igd-passthru=on, but using it for the pc
machine type, xen accelerator, and igd-passtru=on was omitted from that
commit.

The igd-passthru-i440FX pci host device is also needed for guests
configured with the pc machine type, the xen accelerator, and
igd-passthru=on. Specifically, tests show that not using the igd-specific
pci host device with the Intel igd passed through to the guest results
in slower startup performance and reduced resolution of the display
during startup. This patch fixes this issue.

To simplify the logic that is needed to support both the --enable-xen
and the --disable-xen configure options, introduce the boolean symbol
pc_xen_igd_gfx_pt_enabled() whose value is set appropriately in the
sysemu/xen.h header file as the test to determine whether or not
to use the igd-passthrough-i440FX pci host device instead of the
normal i440FX pci host device.

Fixes: 998250e97661 ("xen, gfx passthrough: register host bridge specific to 
passthrough")
Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
---
This patch is intended to replace or complement a recently proposed
patch that modifies slot_reserved_mask for the xenfv machine with
igd-passthru=on in order to fix the problem of Qemu not reserving slot 2
for the Intel IGD for the xenfv machine type. This patch provides a
simple way to improve Qemu support for the Intel IGD with the xen
accelerator without needing to change how slot_reserved_mask functions.

For reference, the latest version of the patch to fix the xenfv machine
using slot_reserved_mask is at:

https://lore.kernel.org/qemu-devel/b1b4a21fe9a600b1322742dda55a40e9961daa57.1674346505.git.brchuckz@xxxxxxx/

Reason for introducing the new boolean pc_xen_igd_gfx_pt_enabled():

It is also possible to use xen_igd_gfx_pt_enabled() directly to check
if the igd-passthru-i440FX pci host device is needed in this patch,
but in that case it would be necessary to implement it in
accel/stubs/xen-stub.c like this:

bool xen_igd_gfx_pt_enabled(void)
{
    return false;
}

to cover the case when Qemu is configured with --disable-xen. I thought
it was simpler to introduce the same boolean prefixed with pc_ and
set it to 0 when --disable-xen is the configure option, and that explains
why the proposed patch introduces pc_xen_igd_gfx_pt_enabled() instead of
using xen_igd_gfx_pt_enabled() directly.

Another reason to use pc_xen_igd_gfx_pt_enabled() is to distinguish it
from xen_igd_gfx_pt_enabled() in hw/i386/pc_piix.c, because the use of
xen_igd_gfx_pt_enabled() is guarded by CONFIG_XEN but this patch needs
to place the boolean in a position that is not guarded by CONFIG_XEN.
This approach will simplify any future effort to move the code in
pc_piix.c that is not guarded by CONFIG_XEN to a xen-specific file.

 hw/i386/pc_piix.c    | 2 ++
 include/sysemu/xen.h | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index df64dd8dcc..fd5b9ae1eb 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -433,6 +433,8 @@ static void pc_xen_hvm_init(MachineState *machine)
             compat(machine); \
         } \
         pc_init1(machine, TYPE_I440FX_PCI_HOST_BRIDGE, \
+                 pc_xen_igd_gfx_pt_enabled() ? \
+                 TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE : \
                  TYPE_I440FX_PCI_DEVICE); \
     } \
     DEFINE_PC_MACHINE(suffix, name, pc_init_##suffix, optionfn)
diff --git a/include/sysemu/xen.h b/include/sysemu/xen.h
index 0ca25697e4..99ae41e619 100644
--- a/include/sysemu/xen.h
+++ b/include/sysemu/xen.h
@@ -23,6 +23,7 @@
 extern bool xen_allowed;
 
 #define xen_enabled()           (xen_allowed)
+#define pc_xen_igd_gfx_pt_enabled()    xen_igd_gfx_pt_enabled()
 
 #ifndef CONFIG_USER_ONLY
 void xen_hvm_modified_memory(ram_addr_t start, ram_addr_t length);
@@ -33,6 +34,7 @@ void xen_ram_alloc(ram_addr_t ram_addr, ram_addr_t size,
 #else /* !CONFIG_XEN_IS_POSSIBLE */
 
 #define xen_enabled() 0
+#define pc_xen_igd_gfx_pt_enabled() 0
 #ifndef CONFIG_USER_ONLY
 static inline void xen_hvm_modified_memory(ram_addr_t start, ram_addr_t length)
 {
-- 
2.39.1