linux-kernel - Re: [RFC PATCH v6 064/104] KVM: TDX: Add helper assembly function to TDX vcpu

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Date:   Tue, 31 May 2022 17:58:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, erdemaktas@...gle.com,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>
Subject: Re: [RFC PATCH v6 064/104] KVM: TDX: Add helper assembly function to
 TDX vcpu

On 5/5/22 20:14, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> TDX defines an API to run TDX vcpu with its own ABI.  Define an assembly
> helper function to run TDX vcpu to hide the special ABI so that C code can
> call it with function call ABI.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>

"ret" needs to be "RET" to support SLS mitigation.

Paolo

> ---
>   arch/x86/kvm/vmx/vmenter.S | 146 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 146 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 435c187927c4..f655bcca0e93 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -2,6 +2,7 @@
>   #include <linux/linkage.h>
>   #include <asm/asm.h>
>   #include <asm/bitsperlong.h>
> +#include <asm/errno.h>
>   #include <asm/kvm_vcpu_regs.h>
>   #include <asm/nospec-branch.h>
>   #include <asm/segment.h>
> @@ -28,6 +29,13 @@
>   #define VCPU_R15	__VCPU_REGS_R15 * WORD_SIZE
>   #endif
>   
> +#ifdef CONFIG_INTEL_TDX_HOST
> +#define TDENTER 		0
> +#define EXIT_REASON_TDCALL	77
> +#define TDENTER_ERROR_BIT	63
> +#define seamcall		.byte 0x66,0x0f,0x01,0xcf
> +#endif
> +
>   .section .noinstr.text, "ax"
>   
>   /**
> @@ -328,3 +336,141 @@ SYM_FUNC_START(vmx_do_interrupt_nmi_irqoff)
>   	pop %_ASM_BP
>   	RET
>   SYM_FUNC_END(vmx_do_interrupt_nmi_irqoff)
> +
> +#ifdef CONFIG_INTEL_TDX_HOST
> +
> +.pushsection .noinstr.text, "ax"
> +
> +/**
> + * __tdx_vcpu_run - Call SEAMCALL(TDENTER) to run a TD vcpu
> + * @tdvpr:	physical address of TDVPR
> + * @regs:	void * (to registers of TDVCPU)
> + * @gpr_mask:	non-zero if guest registers need to be loaded prior to TDENTER
> + *
> + * Returns:
> + *	TD-Exit Reason
> + *
> + * Note: KVM doesn't support using XMM in its hypercalls, it's the HyperV
> + *	 code's responsibility to save/restore XMM registers on TDVMCALL.
> + */
> +SYM_FUNC_START(__tdx_vcpu_run)
> +	push %rbp
> +	mov  %rsp, %rbp
> +
> +	push %r15
> +	push %r14
> +	push %r13
> +	push %r12
> +	push %rbx
> +
> +	/* Save @regs, which is needed after TDENTER to capture output. */
> +	push %rsi
> +
> +	/* Load @tdvpr to RCX */
> +	mov %rdi, %rcx
> +
> +	/* No need to load guest GPRs if the last exit wasn't a TDVMCALL. */
> +	test %dx, %dx
> +	je 1f
> +
> +	/* Load @regs to RAX, which will be clobbered with $TDENTER anyways. */
> +	mov %rsi, %rax
> +
> +	mov VCPU_RBX(%rax), %rbx
> +	mov VCPU_RDX(%rax), %rdx
> +	mov VCPU_RBP(%rax), %rbp
> +	mov VCPU_RSI(%rax), %rsi
> +	mov VCPU_RDI(%rax), %rdi
> +
> +	mov VCPU_R8 (%rax),  %r8
> +	mov VCPU_R9 (%rax),  %r9
> +	mov VCPU_R10(%rax), %r10
> +	mov VCPU_R11(%rax), %r11
> +	mov VCPU_R12(%rax), %r12
> +	mov VCPU_R13(%rax), %r13
> +	mov VCPU_R14(%rax), %r14
> +	mov VCPU_R15(%rax), %r15
> +
> +	/*  Load TDENTER to RAX.  This kills the @regs pointer! */
> +1:	mov $TDENTER, %rax
> +
> +2:	seamcall
> +
> +	/* Skip to the exit path if TDENTER failed. */
> +	bt $TDENTER_ERROR_BIT, %rax
> +	jc 4f
> +
> +	/* Temporarily save the TD-Exit reason. */
> +	push %rax
> +
> +	/* check if TD-exit due to TDVMCALL */
> +	cmp $EXIT_REASON_TDCALL, %ax
> +
> +	/* Reload @regs to RAX. */
> +	mov 8(%rsp), %rax
> +
> +	/* Jump on non-TDVMCALL */
> +	jne 3f
> +
> +	/* Save all output from SEAMCALL(TDENTER) */
> +	mov %rbx, VCPU_RBX(%rax)
> +	mov %rbp, VCPU_RBP(%rax)
> +	mov %rsi, VCPU_RSI(%rax)
> +	mov %rdi, VCPU_RDI(%rax)
> +	mov %r10, VCPU_R10(%rax)
> +	mov %r11, VCPU_R11(%rax)
> +	mov %r12, VCPU_R12(%rax)
> +	mov %r13, VCPU_R13(%rax)
> +	mov %r14, VCPU_R14(%rax)
> +	mov %r15, VCPU_R15(%rax)
> +
> +3:	mov %rcx, VCPU_RCX(%rax)
> +	mov %rdx, VCPU_RDX(%rax)
> +	mov %r8,  VCPU_R8 (%rax)
> +	mov %r9,  VCPU_R9 (%rax)
> +
> +	/*
> +	 * Clear all general purpose registers except RSP and RAX to prevent
> +	 * speculative use of the guest's values.
> +	 */
> +	xor %rbx, %rbx
> +	xor %rcx, %rcx
> +	xor %rdx, %rdx
> +	xor %rsi, %rsi
> +	xor %rdi, %rdi
> +	xor %rbp, %rbp
> +	xor %r8,  %r8
> +	xor %r9,  %r9
> +	xor %r10, %r10
> +	xor %r11, %r11
> +	xor %r12, %r12
> +	xor %r13, %r13
> +	xor %r14, %r14
> +	xor %r15, %r15
> +
> +	/* Restore the TD-Exit reason to RAX for return. */
> +	pop %rax
> +
> +	/* "POP" @regs. */
> +4:	add $8, %rsp
> +	pop %rbx
> +	pop %r12
> +	pop %r13
> +	pop %r14
> +	pop %r15
> +
> +	pop %rbp
> +	ret
> +
> +5:	cmpb $0, kvm_rebooting
> +	je 6f
> +	mov $-EFAULT, %rax
> +	jmp 4b
> +6:	ud2
> +	_ASM_EXTABLE(2b, 5b)
> +
> +SYM_FUNC_END(__tdx_vcpu_run)
> +
> +.popsection
> +
> +#endif