Re: [RFC PATCH V6 02/14] x86/sev: Add Check of #HV event in path

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 15, 2023 at 12:59:04PM -0400, Tianyu Lan wrote:
> From: Tianyu Lan <tiala@microsoft.com>
> 
> Add check_hv_pending() and check_hv_pending_after_irq() to
> check queued #HV event when irq is disabled.
> 
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> ---
>  arch/x86/entry/entry_64.S       | 18 ++++++++++++++++
>  arch/x86/include/asm/irqflags.h | 14 +++++++++++-
>  arch/x86/kernel/sev.c           | 38 +++++++++++++++++++++++++++++++++
>  3 files changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 653b1f10699b..147b850babf6 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1019,6 +1019,15 @@ SYM_CODE_END(paranoid_entry)
>   * R15 - old SPEC_CTRL
>   */
>  SYM_CODE_START_LOCAL(paranoid_exit)
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	/*
> +	 * If a #HV was delivered during execution and interrupts were
> +	 * disabled, then check if it can be handled before the iret
> +	 * (which may re-enable interrupts).
> +	 */
> +	mov     %rsp, %rdi
> +	call    check_hv_pending
> +#endif
>  	UNWIND_HINT_REGS
>  
>  	/*
> @@ -1143,6 +1152,15 @@ SYM_CODE_START(error_entry)
>  SYM_CODE_END(error_entry)
>  
>  SYM_CODE_START_LOCAL(error_return)
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	/*
> +	 * If a #HV was delivered during execution and interrupts were
> +	 * disabled, then check if it can be handled before the iret
> +	 * (which may re-enable interrupts).
> +	 */
> +	mov     %rsp, %rdi
> +	call    check_hv_pending
> +#endif
>  	UNWIND_HINT_REGS
>  	DEBUG_ENTRY_ASSERT_IRQS_OFF
>  	testb	$3, CS(%rsp)

Oh hell no... do now you're adding unconditional calls to every single
interrupt and nmi exit path, with the grand total of 0 justification.