[PATCH v2 1/1] x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index()

[Michael Kelley <mhkelley58@gmail.com>]

hv_apicid_to_vp_index() currently uses the per-cpu hypercall input
and output pages. This function is called when running in VTL2 and
when running in an SEV-SNP CoCo VM with no paravisor. In the former
case, the output page is allocated, but in the latter case it is
not, so the hypervisor stores the output VP index in memory that has
not been allocated by the guest.

Fix this by using the input page for both input and output. The
hypercall has very small input and output, so sharing the same
page for both is straightforward. An alternative fix is to
allocate the per-cpu output page when running in an SEV-SNP CoCo
guest, but this uses significantly more memory, particularly
with larger vCPUs counts.

Fixes: 86c48271e0d6 ("x86/hyperv: Fix APIC ID and VP index confusion in hv_snp_boot_ap()")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
I'm not aware that this bug is causing any real problems because
SEV-SNP CoCo VMs on Hyper-V are rarely, if ever, used without a
paravisor. But it was on my list of little clean-ups to do, and
a recent Sashiko analysis [1] flagged the issue. So the best thing
to do is just fix it.

[1] https://lore.kernel.org/linux-hyperv/20260901171238.834601F00A3A@smtp.kernel.org/

Changes in v2:
* Make the new code a bit cleaner by dropping the unnecessary cast
  to u32 *

 arch/x86/hyperv/hv_init.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index d5edc8530964..2a71c70d7e27 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -731,7 +731,8 @@ int hv_apicid_to_vp_index(u32 apic_id)
 	input->partition_id = HV_PARTITION_ID_SELF;
 	input->apic_ids[0] = apic_id;
 
-	output = *this_cpu_ptr(hyperv_pcpu_output_arg);
+	/* Treat input as having 2 APIC IDs so output is 64-bit aligned */
+	output = (void *)input + struct_size(input, apic_ids, 2);
 
 	control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_INDEX_FROM_APIC_ID;
 	status = hv_do_hypercall(control, input, output);
-- 
2.25.1