Drivers: hv: vmbus: Support >64 VPs for a fully enlightened TDX/SNP VM

Don't set *this_cpu_ptr(hyperv_pcpu_input_arg) before the function
set_memory_decrypted() returns, otherwise we run into this ticky issue:

For a fully enlightened TDX/SNP VM, in hv_common_cpu_init(),
*this_cpu_ptr(hyperv_pcpu_input_arg) is an encrypted page before
the set_memory_decrypted() returns.

When such a VM has more than 64 VPs, if the hyperv_pcpu_input_arg is not
NULL, hv_common_cpu_init() -> set_memory_decrypted() -> ... ->
cpa_flush() -> on_each_cpu() -> ... -> hv_send_ipi_mask() -> ... ->
__send_ipi_mask_ex() tries to call hv_do_rep_hypercall() with the
hyperv_pcpu_input_arg as the hypercall input page, which must be a
decrypted page in such a VM, but the page is still encrypted at this
point, and a fatal fault is triggered.

Fix the issue by setting *this_cpu_ptr(hyperv_pcpu_input_arg) after
set_memory_decrypted(): if the hyperv_pcpu_input_arg is NULL,
__send_ipi_mask_ex() returns HV_STATUS_INVALID_PARAMETER immediately,
and hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
which can use x2apic_send_IPI_all(), which may be slightly slower than
the hypercall but still works correctly in such a VM.

Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20230824080712.30327-6-decui@microsoft.com

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index 897bbb9..4c858e1 100644 (file)
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -360,6 +360,7 @@ int hv_common_cpu_init(unsigned int cpu)
        u64 msr_vp_index;
        gfp_t flags;
        int pgcount = hv_root_partition ? 2 : 1;
+       void *mem;
        int ret;
 
        /* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
@@ -372,25 +373,40 @@ int hv_common_cpu_init(unsigned int cpu)
         * allocated if this CPU was previously online and then taken offline
         */
        if (!*inputarg) {
-               *inputarg = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
-               if (!(*inputarg))
+               mem = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
+               if (!mem)
                        return -ENOMEM;
 
                if (hv_root_partition) {
                        outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
-                       *outputarg = (char *)(*inputarg) + HV_HYP_PAGE_SIZE;
+                       *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
                }
 
                if (hv_isolation_type_en_snp() || hv_isolation_type_tdx()) {
-                       ret = set_memory_decrypted((unsigned long)*inputarg, pgcount);
+                       ret = set_memory_decrypted((unsigned long)mem, pgcount);
                        if (ret) {
-                               /* It may be unsafe to free *inputarg */
-                               *inputarg = NULL;
+                               /* It may be unsafe to free 'mem' */
                                return ret;
                        }
 
-                       memset(*inputarg, 0x00, pgcount * PAGE_SIZE);
+                       memset(mem, 0x00, pgcount * HV_HYP_PAGE_SIZE);
                }
+
+               /*
+                * In a fully enlightened TDX/SNP VM with more than 64 VPs, if
+                * hyperv_pcpu_input_arg is not NULL, set_memory_decrypted() ->
+                * ... -> cpa_flush()-> ... -> __send_ipi_mask_ex() tries to
+                * use hyperv_pcpu_input_arg as the hypercall input page, which
+                * must be a decrypted page in such a VM, but the page is still
+                * encrypted before set_memory_decrypted() returns. Fix this by
+                * setting *inputarg after the above set_memory_decrypted(): if
+                * hyperv_pcpu_input_arg is NULL, __send_ipi_mask_ex() returns
+                * HV_STATUS_INVALID_PARAMETER immediately, and the function
+                * hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
+                * which may be slightly slower than the hypercall, but still
+                * works correctly in such a VM.
+                */
+               *inputarg = mem;
        }
 
        msr_vp_index = hv_get_register(HV_REGISTER_VP_INDEX);