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SubjectRe: [RFC PATCH V3 16/16] x86/sev: Fix interrupt exit code paths from #HV exception
From
On 2/2/2023 5:20 PM, Zhi Wang wrote:
> On Sat, 21 Jan 2023 21:46:06 -0500
> Tianyu Lan <ltykernel@gmail.com> wrote:
>
>> From: Ashish Kalra <ashish.kalra@amd.com>
>>
>> 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.
>>
>
> Why doesn't this problem happen in #VC handler? As #VC handler doesn't have
> this special handling.
>

Because the #VC handler does not invoke common_interrupt() handler to do
IRQ processing. Doing IRQ handling is specific to #HV exception handler
as all guest interrupt handling is invoked from #HV exception handler
once restricted interrupt injection support is enabled.

Thanks,
Ashish

>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
>> ---
>> 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)
>> +{
>> + /*
>> + * 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);
>> +}
>

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