RE: [PATCH v6 5/8] x86/hyperv: Use vmmcall to implement Hyper-V hypercall in sev-snp enlightened guest

Dexuan Cui
2023-08-16T21:24:47Z

> To: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; wei.liu@kernel.org; Dexuan Cui
> [...]
> In sev-snp enlightened guest, Hyper-V hypercall needs
> to use vmmcall to trigger vmexit and notify hypervisor
> to handle hypercall request.
>
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> ---
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -59,16 +59,25 @@ static inline u64 hv_do_hypercall(u64 control, void
> *input, void *output)
> u64 hv_status;
>
> #ifdef CONFIG_X86_64
> - if (!hv_hypercall_pg)
> - return U64_MAX;
> + if (hv_isolation_type_en_snp()) {
> + __asm__ __volatile__("mov %4, %%r8\n"
> + "vmmcall"
> + : "=a" (hv_status),
> ASM_CALL_CONSTRAINT,
> + "+c" (control), "+d" (input_address)
> + : "r" (output_address)
> + : "cc", "memory", "r8", "r9", "r10", "r11");
> + } else {
> + if (!hv_hypercall_pg)
> + return U64_MAX;
>
> - __asm__ __volatile__("mov %4, %%r8\n"
> - CALL_NOSPEC
> - : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> - "+c" (control), "+d" (input_address)
> - : "r" (output_address),
> - THUNK_TARGET(hv_hypercall_pg)
> - : "cc", "memory", "r8", "r9", "r10", "r11");
> + __asm__ __volatile__("mov %4, %%r8\n"
> + CALL_NOSPEC
> + : "=a" (hv_status),
> ASM_CALL_CONSTRAINT,
> + "+c" (control), "+d" (input_address)
> + : "r" (output_address),
> + THUNK_TARGET(hv_hypercall_pg)
> + : "cc", "memory", "r8", "r9", "r10", "r11");
> + }

IMO it's better if we add a "return hv_status;" for the SNP case, and don't move
the assembly code for the regular VM. I made a patch:
https://github.com/dcui/tdx/commit/f81013578605aa02939a3186afa9fc76791b3acd

You may want to explain briefly why the earlier approach
ALTERNATIVE(CALL_NOSPEC, "vmmcall", X86_FEATURE_SEV_ES)
doesn't work:

start_kernel() calls hyperv_init() before alternative_instructions(), and hyperv_init()
already uses hypercalls, e.g. the newly-added get_vtl() in your patch 2.

start_kernel:
late_time_init
x86_late_time_init
x86_init.irqs.intr_mode_init
apic_intr_mode_init
x86_platform.apic_post_init
hyperv_init ==> it already uses hypercalls, e.g. the newly-added get_vtl() in your patch 2.

arch_cpu_finalize_init()
alternative_instructions()

We can move the get_vtl hypercall to a later place, but there are also
other hypercalls before alternative_instructions(). IMO it may be unsafe
to run ALTERNATIVE code before alternative_instructions() is called.