Re: [RFC PATCH V3 16/16] x86/sev: Fix interrupt exit code paths from #HV exception

["Gupta, Pankaj" <pankaj.gupta@xxxxxxx>]

On 1/22/2023 3:46 AM, Tianyu Lan wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> Add checks in interrupt exit code paths in case of returns
> to user mode to check if currently executing the #HV handler
> then don't follow the irqentry_exit_to_user_mode path as
> that can potentially cause the #HV handler to be
> preempted and rescheduled on another CPU. Rescheduled #HV
> handler on another cpu will cause interrupts to be handled
> on a different cpu than the injected one, causing
> invalid EOIs and missed/lost guest interrupts and
> corresponding hangs and/or per-cpu IRQs handled on
> non-intended cpu.
>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>   arch/x86/include/asm/idtentry.h | 66 +++++++++++++++++++++++++++++++++
>   arch/x86/kernel/sev.c           | 30 +++++++++++++++
>   2 files changed, 96 insertions(+)
>
> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
> index 652fea10d377..45b47132be7c 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -13,6 +13,10 @@
>   
>   #include <asm/irq_stack.h>
>   
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +noinstr void irqentry_exit_hv_cond(struct pt_regs *regs, irqentry_state_t state);
> +#endif
> +
>   /**
>    * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
>    *		      No error code pushed by hardware
> @@ -176,6 +180,7 @@ __visible noinstr void func(struct pt_regs *regs, unsigned long error_code)
>   #define DECLARE_IDTENTRY_IRQ(vector, func)				\
>   	DECLARE_IDTENTRY_ERRORCODE(vector, func)
>   
> +#ifndef CONFIG_AMD_MEM_ENCRYPT
>   /**
>    * DEFINE_IDTENTRY_IRQ - Emit code for device interrupt IDT entry points
>    * @func:	Function name of the entry point
> @@ -205,6 +210,26 @@ __visible noinstr void func(struct pt_regs *regs,			\
>   }									\
>   									\
>   static noinline void __##func(struct pt_regs *regs, u32 vector)
> +#else
> +
> +#define DEFINE_IDTENTRY_IRQ(func)					\
> +static void __##func(struct pt_regs *regs, u32 vector);		\
> +									\
> +__visible noinstr void func(struct pt_regs *regs,			\
> +			    unsigned long error_code)			\
> +{									\
> +	irqentry_state_t state = irqentry_enter(regs);			\
> +	u32 vector = (u32)(u8)error_code;				\
> +									\
> +	instrumentation_begin();					\
> +	kvm_set_cpu_l1tf_flush_l1d();					\
> +	run_irq_on_irqstack_cond(__##func, regs, vector);		\
> +	instrumentation_end();						\
> +	irqentry_exit_hv_cond(regs, state);				\
> +}									\
> +									\
> +static noinline void __##func(struct pt_regs *regs, u32 vector)
> +#endif
>   
>   /**
>    * DECLARE_IDTENTRY_SYSVEC - Declare functions for system vector entry points
> @@ -221,6 +246,7 @@ static noinline void __##func(struct pt_regs *regs, u32 vector)
>   #define DECLARE_IDTENTRY_SYSVEC(vector, func)				\
>   	DECLARE_IDTENTRY(vector, func)
>   
> +#ifndef CONFIG_AMD_MEM_ENCRYPT
>   /**
>    * DEFINE_IDTENTRY_SYSVEC - Emit code for system vector IDT entry points
>    * @func:	Function name of the entry point
> @@ -245,6 +271,26 @@ __visible noinstr void func(struct pt_regs *regs)			\
>   }									\
>   									\
>   static noinline void __##func(struct pt_regs *regs)
> +#else
> +
> +#define DEFINE_IDTENTRY_SYSVEC(func)					\
> +static void __##func(struct pt_regs *regs);				\
> +									\
> +__visible noinstr void func(struct pt_regs *regs)			\
> +{									\
> +	irqentry_state_t state = irqentry_enter(regs);			\
> +									\
> +	instrumentation_begin();					\
> +	kvm_set_cpu_l1tf_flush_l1d();					\
> +	run_sysvec_on_irqstack_cond(__##func, regs);			\
> +	instrumentation_end();						\
> +	irqentry_exit_hv_cond(regs, state);				\
> +}									\
> +									\
> +static noinline void __##func(struct pt_regs *regs)
> +#endif
> +
> +#ifndef CONFIG_AMD_MEM_ENCRYPT
>   
>   /**
>    * DEFINE_IDTENTRY_SYSVEC_SIMPLE - Emit code for simple system vector IDT
> @@ -274,6 +320,26 @@ __visible noinstr void func(struct pt_regs *regs)			\
>   }									\
>   									\
>   static __always_inline void __##func(struct pt_regs *regs)
> +#else
> +
> +#define DEFINE_IDTENTRY_SYSVEC_SIMPLE(func)				\
> +static __always_inline void __##func(struct pt_regs *regs);		\
> +									\
> +__visible noinstr void func(struct pt_regs *regs)			\
> +{									\
> +	irqentry_state_t state = irqentry_enter(regs);			\
> +									\
> +	instrumentation_begin();					\
> +	__irq_enter_raw();						\
> +	kvm_set_cpu_l1tf_flush_l1d();					\
> +	__##func(regs);						\
> +	__irq_exit_raw();						\
> +	instrumentation_end();						\
> +	irqentry_exit_hv_cond(regs, state);				\
> +}									\
> +									\
> +static __always_inline void __##func(struct pt_regs *regs)
> +#endif
>   
>   /**
>    * DECLARE_IDTENTRY_XENCB - Declare functions for XEN HV callback entry point
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index b1a98c2a52f8..23f15e95838b 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -147,6 +147,10 @@ struct sev_hv_doorbell_page {
>   
>   struct sev_snp_runtime_data {
>   	struct sev_hv_doorbell_page hv_doorbell_page;
> +	/*
> +	 * Indication that we are currently handling #HV events.
> +	 */
> +	bool hv_handling_events;
>   };
>   
>   static DEFINE_PER_CPU(struct sev_snp_runtime_data*, snp_runtime_data);
> @@ -200,6 +204,8 @@ static void do_exc_hv(struct pt_regs *regs)
>   	union hv_pending_events pending_events;
>   	u8 vector;
>   
> +	this_cpu_read(snp_runtime_data)->hv_handling_events = true;
> +
>   	while (sev_hv_pending()) {
>   		pending_events.events = xchg(
>   			&sev_snp_current_doorbell_page()->pending_events.events,
> @@ -234,6 +240,8 @@ static void do_exc_hv(struct pt_regs *regs)
>   			common_interrupt(regs, pending_events.vector);
>   		}
>   	}
> +
> +	this_cpu_read(snp_runtime_data)->hv_handling_events = false;
>   }
>   
>   static __always_inline bool on_vc_stack(struct pt_regs *regs)
> @@ -2529,3 +2537,25 @@ static int __init snp_init_platform_device(void)
>   	return 0;
>   }
>   device_initcall(snp_init_platform_device);
> +
> +noinstr void irqentry_exit_hv_cond(struct pt_regs *regs, irqentry_state_t state)
> +{

This code path is being called even for the guest without SNP. Ran
a SEV guest and guest crashed in this code path. Checking & returning
made guest (non SNP) to boot with some call traces. But this branch 
needs to be avoided for non-SNP guests and host as well.

Thanks,
Pankaj

+++ b/arch/x86/kernel/sev.c
@@ -2540,6 +2540,9 @@ device_initcall(snp_init_platform_device);

 noinstr void irqentry_exit_hv_cond(struct pt_regs *regs, 
irqentry_state_t state)
 {
+
+       if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
+                               return;

> +	/*
> +	 * Check whether this returns to user mode, if so and if
> +	 * we are currently executing the #HV handler then we don't
> +	 * want to follow the irqentry_exit_to_user_mode path as
> +	 * that can potentially cause the #HV handler to be
> +	 * preempted and rescheduled on another CPU. Rescheduled #HV
> +	 * handler on another cpu will cause interrupts to be handled
> +	 * on a different cpu than the injected one, causing
> +	 * invalid EOIs and missed/lost guest interrupts and
> +	 * corresponding hangs and/or per-cpu IRQs handled on
> +	 * non-intended cpu.
> +	 */
> +	if (user_mode(regs) &&
> +	    this_cpu_read(snp_runtime_data)->hv_handling_events)
> +		return;
> +
> +	/* follow normal interrupt return/exit path */
> +	irqentry_exit(regs, state);
> +}