[PATCH 6.6 059/638] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

[Sasha Levin]

From: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>

[ Upstream commit 0d63e4c0ebc2b5c329babde44fd61d3f08db814d ]

The current method for signaling the compatibility of a Hyper-V host
with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
However, for higher VTLs, this leaf is not reported, due to the absence
of an IO-APIC.

As an alternative, assume that when running at a high VTL, the host
supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
employ any architectural MSIs at higher VTLs

This unblocks startup of VTL2 environments with more than 256 CPUs.

Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/1705341460-18394-1-git-send-email-ssengar@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
Message-ID: <1705341460-18394-1-git-send-email-ssengar@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/hyperv/hv_vtl.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 999f5ac82fe90..53b309d41b3b9 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -16,6 +16,11 @@
 extern struct boot_params boot_params;
 static struct real_mode_header hv_vtl_real_mode_header;
 
+static bool __init hv_vtl_msi_ext_dest_id(void)
+{
+	return true;
+}
+
 void __init hv_vtl_init_platform(void)
 {
 	pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
@@ -38,6 +43,8 @@ void __init hv_vtl_init_platform(void)
 	x86_platform.legacy.warm_reset = 0;
 	x86_platform.legacy.reserve_bios_regions = 0;
 	x86_platform.legacy.devices.pnpbios = 0;
+
+	x86_init.hyper.msi_ext_dest_id = hv_vtl_msi_ext_dest_id;
 }
 
 static inline u64 hv_vtl_system_desc_base(struct ldttss_desc *desc)
-- 
2.43.0