From e8e9ed3790feac05ec6fd5c279ec80b05293d67b Mon Sep 17 00:00:00 2001 From: Evgenii Shatokhin Date: Mon, 6 Mar 2023 00:22:46 +0300 Subject: lib: sbi: Set the state of a hart to START_PENDING after the hart is ready When a boot hart executes sbi_hsm_hart_start() to start a secondary hart, next_arg1, next_addr and next_mode for the latter are stored in the scratch area after the state has been set to SBI_HSM_STATE_START_PENDING. The secondary hart waits in the loop with wfi() in sbi_hsm_hart_wait() at that time. However, "wfi" instruction is not guaranteed to wait for an interrupt to be received by the hart, it is just a hint for the CPU. According to RISC-V Privileged Architectures spec. v20211203, even an implementation of "wfi" as "nop" is legal. So, the secondary might leave the loop in sbi_hsm_hart_wait() as soon as its state has been set to SBI_HSM_STATE_START_PENDING, even if it got no IPI or it got an IPI unrelated to sbi_hsm_hart_start(). This could lead to the following race condition when booting Linux, for example: Boot hart (#0) Secondary hart (#1) runs Linux startup code waits in sbi_hsm_hart_wait() sbi_ecall(SBI_EXT_HSM, SBI_EXT_HSM_HART_START, ...) enters sbi_hsm_hart_start() sets state of hart #1 to START_PENDING leaves sbi_hsm_hart_wait() runs to the end of init_warmboot() returns to scratch->next_addr (next_addr can be garbage here) sets next_addr, etc. for hart #1 (no good: hart #1 has already left) sends IPI to hart #1 (no good either) If this happens, the secondary hart jumps to a wrong next_addr at the end of init_warmboot(), which leads to a system hang or crash. To reproduce the issue more reliably, one could add a delay in sbi_hsm_hart_start() after setting the hart's state but before sending IPI to that hart: hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED, SBI_HSM_STATE_START_PENDING); ... + sbi_timer_mdelay(10); init_count = sbi_init_count(hartid); rscratch->next_arg1 = arg1; rscratch->next_addr = saddr; The issue can be reproduced, for example, in a QEMU VM with '-machine virt' and 2 or more CPUs, with Linux as the guest OS. This patch moves writing of next_arg1, next_addr and next_mode for the secondary hart before setting its state to SBI_HSM_STATE_START_PENDING. In theory, it is possible that two or more harts enter sbi_hsm_hart_start() for the same target hart simultaneously. To make sure the current hart has exclusive access to the scratch area of the target hart at that point, a per-hart 'start_ticket' is used. It is initially 0. The current hart tries to acquire the ticket first (set it to 1) at the beginning of sbi_hsm_hart_start() and only proceeds if it has successfully acquired it. The target hart reads next_addr, etc., and then the releases the ticket (sets it to 0) before calling sbi_hart_switch_mode(). This way, even if some other hart manages to enter sbi_hsm_hart_start() after the ticket has been released but before the target hart jumps to next_addr, it will not cause problems. atomic_cmpxchg() already has "acquire" semantics, among other things, so no additional barriers are needed in hsm_start_ticket_acquire(). No hart can perform or observe the update of *rscratch before setting of 'start_ticket' to 1. atomic_write() only imposes ordering of writes, so an explicit barrier is needed in hsm_start_ticket_release() to ensure its "release" semantics. This guarantees that reads of scratch->next_addr, etc., in sbi_hsm_hart_start_finish() cannot happen after 'start_ticket' has been released. Signed-off-by: Evgenii Shatokhin Reviewed-by: Anup Patel --- lib/sbi/sbi_hsm.c | 83 ++++++++++++++++++++++++++++++++++++++++++++----------- 1 file changed, 67 insertions(+), 16 deletions(-) diff --git a/lib/sbi/sbi_hsm.c b/lib/sbi/sbi_hsm.c index 100b8c0..16e3846 100644 --- a/lib/sbi/sbi_hsm.c +++ b/lib/sbi/sbi_hsm.c @@ -44,6 +44,7 @@ struct sbi_hsm_data { unsigned long suspend_type; unsigned long saved_mie; unsigned long saved_mip; + atomic_t start_ticket; }; bool sbi_hsm_hart_change_state(struct sbi_scratch *scratch, long oldstate, @@ -75,6 +76,32 @@ int sbi_hsm_hart_get_state(const struct sbi_domain *dom, u32 hartid) return __sbi_hsm_hart_get_state(hartid); } +/* + * Try to acquire the ticket for the given target hart to make sure only + * one hart prepares the start of the target hart. + * Returns true if the ticket has been acquired, false otherwise. + * + * The function has "acquire" semantics: no memory operations following it + * in the current hart can be seen before it by other harts. + * atomic_cmpxchg() provides the memory barriers needed for that. + */ +static bool hsm_start_ticket_acquire(struct sbi_hsm_data *hdata) +{ + return (atomic_cmpxchg(&hdata->start_ticket, 0, 1) == 0); +} + +/* + * Release the ticket for the given target hart. + * + * The function has "release" semantics: no memory operations preceding it + * in the current hart can be seen after it by other harts. + */ +static void hsm_start_ticket_release(struct sbi_hsm_data *hdata) +{ + RISCV_FENCE(rw, w); + atomic_write(&hdata->start_ticket, 0); +} + /** * Get ulong HART mask for given HART base ID * @param dom the domain to be used for output HART mask @@ -113,6 +140,9 @@ int sbi_hsm_hart_interruptible_mask(const struct sbi_domain *dom, void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch, u32 hartid) { + unsigned long next_arg1; + unsigned long next_addr; + unsigned long next_mode; struct sbi_hsm_data *hdata = sbi_scratch_offset_ptr(scratch, hart_data_offset); @@ -120,8 +150,12 @@ void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch, SBI_HSM_STATE_STARTED)) sbi_hart_hang(); - sbi_hart_switch_mode(hartid, scratch->next_arg1, scratch->next_addr, - scratch->next_mode, false); + next_arg1 = scratch->next_arg1; + next_addr = scratch->next_addr; + next_mode = scratch->next_mode; + hsm_start_ticket_release(hdata); + + sbi_hart_switch_mode(hartid, next_arg1, next_addr, next_mode, false); } static void sbi_hsm_hart_wait(struct sbi_scratch *scratch, u32 hartid) @@ -226,6 +260,7 @@ int sbi_hsm_init(struct sbi_scratch *scratch, u32 hartid, bool cold_boot) (i == hartid) ? SBI_HSM_STATE_START_PENDING : SBI_HSM_STATE_STOPPED); + ATOMIC_INIT(&hdata->start_ticket, 0); } } else { sbi_hsm_hart_wait(scratch, hartid); @@ -270,6 +305,7 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch, unsigned int hstate; struct sbi_scratch *rscratch; struct sbi_hsm_data *hdata; + int rc; /* For now, we only allow start mode to be S-mode or U-mode. */ if (smode != PRV_S && smode != PRV_U) @@ -283,34 +319,49 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch, rscratch = sbi_hartid_to_scratch(hartid); if (!rscratch) return SBI_EINVAL; + hdata = sbi_scratch_offset_ptr(rscratch, hart_data_offset); + if (!hsm_start_ticket_acquire(hdata)) + return SBI_EINVAL; + + init_count = sbi_init_count(hartid); + rscratch->next_arg1 = arg1; + rscratch->next_addr = saddr; + rscratch->next_mode = smode; + + /* + * atomic_cmpxchg() is an implicit barrier. It makes sure that + * other harts see reading of init_count and writing to *rscratch + * before hdata->state is set to SBI_HSM_STATE_START_PENDING. + */ hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED, SBI_HSM_STATE_START_PENDING); - if (hstate == SBI_HSM_STATE_STARTED) - return SBI_EALREADY; + if (hstate == SBI_HSM_STATE_STARTED) { + rc = SBI_EALREADY; + goto err; + } /** * if a hart is already transition to start or stop, another start call * is considered as invalid request. */ - if (hstate != SBI_HSM_STATE_STOPPED) - return SBI_EINVAL; - - init_count = sbi_init_count(hartid); - rscratch->next_arg1 = arg1; - rscratch->next_addr = saddr; - rscratch->next_mode = smode; + if (hstate != SBI_HSM_STATE_STOPPED) { + rc = SBI_EINVAL; + goto err; + } if (hsm_device_has_hart_hotplug() || (hsm_device_has_hart_secondary_boot() && !init_count)) { - return hsm_device_hart_start(hartid, scratch->warmboot_addr); + rc = hsm_device_hart_start(hartid, scratch->warmboot_addr); } else { - int rc = sbi_ipi_raw_send(hartid); - if (rc) - return rc; + rc = sbi_ipi_raw_send(hartid); } - return 0; + if (!rc) + return 0; +err: + hsm_start_ticket_release(hdata); + return rc; } int sbi_hsm_hart_stop(struct sbi_scratch *scratch, bool exitnow) -- cgit v1.2.3