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Keep the write() submission probe's completion record off the stack #47

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Keep the write() submission probe's completion record off the stack #47

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48 changes: 38 additions & 10 deletions dto.c
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@
#include "dto.h"
#include <x86intrin.h>
#include <sched.h>
#include <time.h>
#include <sys/stat.h>
#include <stdatomic.h>
#include <stdbool.h>
Expand Down Expand Up @@ -93,6 +94,9 @@ struct dto_wq {
int wq_fd;
void *wq_portal;
bool wq_mmapped;
/* completion record of the write() submission probe; must outlive
* test_write_syscall() because the device may complete the NOOP late */
struct dsa_completion_record probe_comp __attribute__((aligned(32)));
};

#ifdef DTO_NUMA_SUPPORT
Expand Down Expand Up @@ -929,27 +933,51 @@ static void cleanup_devices() {
}
#endif

/* How long the write() probe waits for its NOOP to complete. The queue is
* shared: behind a burst of large descriptors from other processes a NOOP can
* take many milliseconds, far more than the few thousand pauses the probe
* used to spin. */
#define DTO_PROBE_TIMEOUT_MS 1000

/* Check that the work queue accepts descriptors through write() (the portal
* mmap is refused when idxd needs CAP_SYS_RAWIO for user submission).
*
* The completion record lives in the wq, not on the stack: the device owns
* the NOOP until it completes, and on a saturated queue that can be long
* after this function gave up waiting. With a stack record the late
* completion write landed in whatever frame had reused the slot (a stack
* protector canary, argv parsing state, a pointer), crashing the process
* at startup in ways that looked unrelated. */
static bool test_write_syscall(struct dto_wq *wq)
{
struct dsa_hw_desc desc = {0};
struct dsa_completion_record comp __attribute__((aligned(32)));
int retry = 0;
volatile uint8_t *status = &wq->probe_comp.status;
struct timespec t0, t;
unsigned int spins = 0;

desc.opcode = DSA_OPCODE_NOOP;
desc.flags = IDXD_OP_FLAG_CRAV | IDXD_OP_FLAG_RCR;
comp.status = 0;
*status = 0;

desc.completion_addr = (unsigned long)&comp;
desc.completion_addr = (unsigned long)&wq->probe_comp;

if (dsa_submit(wq, &desc) == SUCCESS) {
while (comp.status == 0 && retry++ < 10000)
_mm_pause();
if (dsa_submit(wq, &desc) != SUCCESS)
return false;

if (comp.status == DSA_COMP_SUCCESS)
return true;
clock_gettime(CLOCK_MONOTONIC, &t0);
while (*status == 0) {
_mm_pause();
if ((++spins & 1023) != 0)
continue;
clock_gettime(CLOCK_MONOTONIC, &t);
if ((t.tv_sec - t0.tv_sec) * 1000 + (t.tv_nsec - t0.tv_nsec) / 1000000 >= DTO_PROBE_TIMEOUT_MS) {
LOG_ERROR("DSA wq %s: write() probe did not complete in %d ms\n",
wq->wq_path, DTO_PROBE_TIMEOUT_MS);
return false;
}
}

return false;
return *status == DSA_COMP_SUCCESS;
}

static int dsa_init_from_wq_list(char *wq_list)
Expand Down
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