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    SubjectRe: [EXTERNAL] Re: [PATCH 5/9] x86/hyperv: Use vmmcall to implement Hyper-V hypercall in sev-snp enlightened guest
    On Tue, Jun 27, 2023 at 06:57:28PM +0800, Tianyu Lan wrote:

    > > "There is no x86 SEV SNP feature(X86_FEATURE_SEV_SNP) flag

    I'm sure we can arrange such a feature if we need it, this isn't rocket
    science. Boris?

    > > support so far and hardware provides MSR_AMD64_SEV register
    > > to check SEV-SNP capability with MSR_AMD64_SEV_ENABLED bit
    > > ALTERNATIVE can't work without SEV-SNP x86 feature flag."
    > > There is no cpuid leaf bit to check AMD SEV-SNP feature.
    > >
    > > After some Hyper-V doesn't provides SEV and SEV-ES guest before and so
    > > may reuse X86_FEATURE_SEV and X86_FEATURE_SEV_ES flag as alternative
    > > feature check for Hyper-V SEV-SNP guest. Will refresh patch.
    > >
    >
    > Hi Peter:
    > I tried using alternative for "vmmcall" and CALL_NOSPEC in a single
    > Inline assembly. The output is different in the SEV-SNP mode. When SEV-
    > SNP is enabled, thunk_target is not required. While it's necessary in
    > the non SEV-SNP mode. Do you have any idea how to differentiate outputs in
    > the single Inline assembly which just like alternative works for
    > assembler template.

    This seems to work; it's a bit magical for having a nested ALTERNATIVE
    but the output seems correct (the outer alternative comes last in
    .altinstructions and thus has precedence). Sure the [thunk_target] input
    goes unsed in one of the alteratives, but who cares.


    static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
    {
    u64 input_address = input ? virt_to_phys(input) : 0;
    u64 output_address = output ? virt_to_phys(output) : 0;
    u64 hv_status;

    #ifdef CONFIG_X86_64
    if (!hv_hypercall_pg)
    return U64_MAX;

    #if 0
    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 {
    __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");
    }
    #endif

    asm volatile("mov %[output], %%r8\n"
    ALTERNATIVE(CALL_NOSPEC, "vmmcall", X86_FEATURE_SEV_ES)
    : "=a" (hv_status), ASM_CALL_CONSTRAINT,
    "+c" (control), "+d" (input_address)
    : [output] "r" (output_address),
    THUNK_TARGET(hv_hypercall_pg)
    : "cc", "memory", "r8", "r9", "r10", "r11");

    #else
    u32 input_address_hi = upper_32_bits(input_address);
    u32 input_address_lo = lower_32_bits(input_address);
    u32 output_address_hi = upper_32_bits(output_address);
    u32 output_address_lo = lower_32_bits(output_address);

    if (!hv_hypercall_pg)
    return U64_MAX;

    __asm__ __volatile__(CALL_NOSPEC
    : "=A" (hv_status),
    "+c" (input_address_lo), ASM_CALL_CONSTRAINT
    : "A" (control),
    "b" (input_address_hi),
    "D"(output_address_hi), "S"(output_address_lo),
    THUNK_TARGET(hv_hypercall_pg)
    : "cc", "memory");
    #endif /* !x86_64 */
    return hv_status;
    }

    (in actual fact x86_64-defconfig + kvm_guest.config + HYPERV)

    $ ./scripts/objdump-func defconfig-build/arch/x86/hyperv/mmu.o hv_do_hypercall
    0000 0000000000000cd0 <hv_do_hypercall.constprop.0>:
    0000 cd0: 48 89 f9 mov %rdi,%rcx
    0003 cd3: 31 d2 xor %edx,%edx
    0005 cd5: 48 85 f6 test %rsi,%rsi
    0008 cd8: 74 1b je cf5 <hv_do_hypercall.constprop.0+0x25>
    000a cda: b8 00 00 00 80 mov $0x80000000,%eax
    000f cdf: 48 01 c6 add %rax,%rsi
    0012 ce2: 72 38 jb d1c <hv_do_hypercall.constprop.0+0x4c>
    0014 ce4: 48 c7 c2 00 00 00 80 mov $0xffffffff80000000,%rdx
    001b ceb: 48 2b 15 00 00 00 00 sub 0x0(%rip),%rdx # cf2 <hv_do_hypercall.constprop.0+0x22> cee: R_X86_64_PC32 page_offset_base-0x4
    0022 cf2: 48 01 f2 add %rsi,%rdx
    0025 cf5: 48 8b 35 00 00 00 00 mov 0x0(%rip),%rsi # cfc <hv_do_hypercall.constprop.0+0x2c> cf8: R_X86_64_PC32 hv_hypercall_pg-0x4
    002c cfc: 48 85 f6 test %rsi,%rsi
    002f cff: 74 0f je d10 <hv_do_hypercall.constprop.0+0x40>
    0031 d01: 31 c0 xor %eax,%eax
    0033 d03: 49 89 c0 mov %rax,%r8
    0036 d06: ff d6 call *%rsi
    0038 d08: 90 nop
    0039 d09: 90 nop
    003a d0a: 90 nop
    003b d0b: e9 00 00 00 00 jmp d10 <hv_do_hypercall.constprop.0+0x40> d0c: R_X86_64_PLT32 __x86_return_thunk-0x4
    0040 d10: 48 c7 c0 ff ff ff ff mov $0xffffffffffffffff,%rax
    0047 d17: e9 00 00 00 00 jmp d1c <hv_do_hypercall.constprop.0+0x4c> d18: R_X86_64_PLT32 __x86_return_thunk-0x4
    004c d1c: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # d23 <hv_do_hypercall.constprop.0+0x53> d1f: R_X86_64_PC32 phys_base-0x4
    0053 d23: eb cd jmp cf2 <hv_do_hypercall.constprop.0+0x22>

    $ objdump -wdr -j .altinstr_replacement defconfig-build/arch/x86/hyperv/mmu.o
    0000000000000000 <.altinstr_replacement>:
    0: f3 48 0f b8 c7 popcnt %rdi,%rax
    5: e8 00 00 00 00 call a <.altinstr_replacement+0xa> 6: R_X86_64_PLT32 __x86_indirect_thunk_rsi-0x4
    a: 0f ae e8 lfence
    d: ff d6 call *%rsi
    f: 0f 01 d9 vmmcall

    $ ./readelf-section.sh defconfig-build/arch/x86/hyperv/mmu.o altinstructions
    Relocation section '.rela.altinstructions' at offset 0x5420 contains 8 entries:
    Offset Info Type Symbol's Value Symbol's Name + Addend
    0000000000000000 0000000200000002 R_X86_64_PC32 0000000000000000 .text + 1e3
    0000000000000004 0000000700000002 R_X86_64_PC32 0000000000000000 .altinstr_replacement + 0
    000000000000000e 0000000200000002 R_X86_64_PC32 0000000000000000 .text + d06
    0000000000000012 0000000700000002 R_X86_64_PC32 0000000000000000 .altinstr_replacement + 5
    000000000000001c 0000000200000002 R_X86_64_PC32 0000000000000000 .text + d06
    0000000000000020 0000000700000002 R_X86_64_PC32 0000000000000000 .altinstr_replacement + a
    000000000000002a 0000000200000002 R_X86_64_PC32 0000000000000000 .text + d06
    000000000000002e 0000000700000002 R_X86_64_PC32 0000000000000000 .altinstr_replacement + f

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