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    Subject[PATCH v3 12/12] x86/virt/tdx: Adjust 'struct tdx_module_args' to use x86 "register index" layout
    Date
    For TDX guest, KVM needs to call __seamcall_saved_ret() to make the
    TDH.VP.ENTER SEAMCALL to enter the guest, possibly taking all registers
    in 'struct tdx_module_args' as input/output.

    KVM caches guest's GPRs in 'kvm_vcpu_arch::regs[]', which follows the
    "register index" hardware layout of x86 GPRs. On the other hand, the
    __seamcall_saved_ret() takes the pointer of 'struct tdx_module_args' as
    argument, thus there's a mismatch.

    KVM could choose to copy input registers from 'vcpu::regs[]' to a
    'struct tdx_module_args' and use that as argument to make the SEAMCALL,
    but such memory copy isn't desired and should be avoided if possible.

    It's not feasible to change KVM's 'vcpu::regs[]' layout due to various
    reasons (e.g., emulation code uses decoded register index as array index
    to access the register). Therefore, adjust 'struct tdx_module_args' to
    match KVM's 'vcpu::regs[]' layout so that KVM can simply do below:

    __seamcall_saved_ret(TDH_VP_ENTER,
    (struct tdx_module_args *)vcpu->arch.regs);

    Note RAX/RSP/RBP are not used by the TDX_MODULE_CALL assembly, but they
    are necessary in order match the layout of 'struct tdx_module_args' to
    KVM's 'vcpu::regs[]'. Thus add them to the structure, but name them as
    *_unused. Also don't include them to asm-offset.c so that any misuse of
    them in the assembly would result in build error.

    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sean Christopherson <seanjc@google.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Isaku Yamahata <isaku.yamahata@intel.com>
    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Kai Huang <kai.huang@intel.com>
    ---

    v2 -> v3:
    - New patch

    ---
    arch/x86/include/asm/shared/tdx.h | 19 +++++++++++++------
    arch/x86/kernel/asm-offsets.c | 6 +++---
    2 files changed, 16 insertions(+), 9 deletions(-)

    diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
    index 74fc466dfdcd..8d1427562c63 100644
    --- a/arch/x86/include/asm/shared/tdx.h
    +++ b/arch/x86/include/asm/shared/tdx.h
    @@ -58,24 +58,31 @@
    * Used in __tdcall*() to gather the input/output registers' values of the
    * TDCALL instruction when requesting services from the TDX module. This is a
    * software only structure and not part of the TDX module/VMM ABI
    + *
    + * Note those *_unused are not used by the TDX_MODULE_CALL assembly.
    + * The layout of this structure also matches KVM's kvm_vcpu_arch::regs[]
    + * layout, which follows the "register index" order of x86 GPRs. KVM
    + * then can simply type cast kvm_vcpu_arch::regs[] to this structure to
    + * avoid the extra memory copy between two structures when making
    + * TDH.VP.ENTER SEAMCALL.
    */
    struct tdx_module_args {
    - /* callee-clobbered */
    + u64 rax_unused;
    u64 rcx;
    u64 rdx;
    + u64 rbx;
    + u64 rsp_unused;
    + u64 rbp_unused;
    + u64 rsi;
    + u64 rdi;
    u64 r8;
    u64 r9;
    - /* extra callee-clobbered */
    u64 r10;
    u64 r11;
    - /* callee-saved + rdi/rsi */
    u64 r12;
    u64 r13;
    u64 r14;
    u64 r15;
    - u64 rbx;
    - u64 rdi;
    - u64 rsi;
    };

    /* Used to communicate with the TDX module */
    diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
    index 6913b372ccf7..e4ad822d3acd 100644
    --- a/arch/x86/kernel/asm-offsets.c
    +++ b/arch/x86/kernel/asm-offsets.c
    @@ -70,6 +70,9 @@ static void __used common(void)
    BLANK();
    OFFSET(TDX_MODULE_rcx, tdx_module_args, rcx);
    OFFSET(TDX_MODULE_rdx, tdx_module_args, rdx);
    + OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
    + OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);
    + OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
    OFFSET(TDX_MODULE_r8, tdx_module_args, r8);
    OFFSET(TDX_MODULE_r9, tdx_module_args, r9);
    OFFSET(TDX_MODULE_r10, tdx_module_args, r10);
    @@ -78,9 +81,6 @@ static void __used common(void)
    OFFSET(TDX_MODULE_r13, tdx_module_args, r13);
    OFFSET(TDX_MODULE_r14, tdx_module_args, r14);
    OFFSET(TDX_MODULE_r15, tdx_module_args, r15);
    - OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
    - OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
    - OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);

    BLANK();
    OFFSET(BP_scratch, boot_params, scratch);
    --
    2.41.0
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