RE: [PATCH 5/6] x86/hyperv: Support hypercalls for TDX guests

[Dexuan Cui <decui@microsoft.com>]

> From: Dave Hansen <dave.hansen@intel.com>
> Sent: Monday, November 28, 2022 11:11 AM
> On 11/28/22 11:03, Dexuan Cui wrote:
> ...
> > u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2)
> > {
> >         struct tdx_hypercall_args args = { };
> >
> >         if (!(control & HV_HYPERCALL_FAST_BIT)) {
> >                 if (param1)
> >                         param1 = cc_mkdec(param1);
> >
> >                 if (param2)
> >                         param2 = cc_mkdec(param2);
> >         }
> >
> >         args.r10 = control;
> >         args.rdx = param1;
> >         args.r8  = param2;
> >
> >         (void)__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);
> >
> >         return args.r11;
> > }
> 
> I still think this is problematic.
> 
> The cc_mkdec() should be done on the parameters when the code still
> *knows* that they are addresses.

Makes sense.
 
> How do we know, for instance, that no hypercall using this interface
> will *ever* take the 0x0 physical address as an argument?

A 0x0 physical address as an argument still works: the 0 is passed
to the hypervisor using GHCI. I believe Hyper-V interprets the 0 as
an error (if the param is needed), and returns an "invalid parameter"
error code to the guest.

Here is the updated version:

diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index 70170049be2c..41395891d112 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -242,6 +242,20 @@ bool hv_isolation_type_tdx(void)
 {
        return static_branch_unlikely(&isolation_type_tdx);
 }
+
+u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2)
+{
+       struct tdx_hypercall_args args = { };
+
+       args.r10 = control;
+       args.rdx = param1;
+       args.r8  = param2;
+
+       (void)__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);
+
+       return args.r11;
+}
+EXPORT_SYMBOL_GPL(hv_tdx_hypercall);
 #endif

 /*
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 9cc6db45c3bc..ea053af067a2 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -10,6 +10,7 @@
 #include <asm/nospec-branch.h>
 #include <asm/paravirt.h>
 #include <asm/mshyperv.h>
+#include <asm/coco.h>

 union hv_ghcb;

@@ -39,6 +40,8 @@ int hv_call_deposit_pages(int node, u64 partition_id, u32 num_pages);
 int hv_call_add_logical_proc(int node, u32 lp_index, u32 acpi_id);
 int hv_call_create_vp(int node, u64 partition_id, u32 vp_index, u32 flags);

+u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2);
+
 static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
 {
        u64 input_address = input ? virt_to_phys(input) : 0;
@@ -46,6 +49,12 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
        u64 hv_status;

 #ifdef CONFIG_X86_64
+       if (hv_isolation_type_tdx()) {
+               u64 param1 = input_address  ? cc_mkdec(input_address)  : 0;
+               u64 param2 = output_address ? cc_mkdec(output_address) : 0;
+               return hv_tdx_hypercall(control, param1, param2);
+       }
+
        if (!hv_hypercall_pg)
                return U64_MAX;

@@ -83,6 +92,9 @@ static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
        u64 hv_status, control = (u64)code | HV_HYPERCALL_FAST_BIT;

 #ifdef CONFIG_X86_64
+       if (hv_isolation_type_tdx())
+               return hv_tdx_hypercall(control, input1, 0);
+
        {
                __asm__ __volatile__(CALL_NOSPEC
                                     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
@@ -114,6 +126,9 @@ static inline u64 hv_do_fast_hypercall16(u16 code, u64 input1, u64 input2)
        u64 hv_status, control = (u64)code | HV_HYPERCALL_FAST_BIT;

 #ifdef CONFIG_X86_64
+       if (hv_isolation_type_tdx())
+               return hv_tdx_hypercall(control, input1, input2);
+
        {
                __asm__ __volatile__("mov %4, %%r8\n"
                                     CALL_NOSPEC