On Wed, Jun 11, 2025 at 04:25:24PM +0000, Orlov, Ivan wrote: > The current implementation of timeout detection works in the following > way: > > 1. Read completion status. If completed, return the data > 2. Sleep for some time (usleep_range) > 3. Check for timeout using current jiffies value. Return an error if > timed out > 4. Goto 1 > > usleep_range doesn't guarantee it's always going to wake up strictly in > (min, max) range, so such a situation is possible: > > 1. Driver reads completion status. No completion yet > 2. Process sleeps indefinitely. In the meantime, TPM responds > 3. We check for timeout without checking for the completion again. > Result is lost. > > Such a situation also happens for the guest VMs: if vCPU goes to sleep > and doesn't get scheduled for some time, the guest TPM driver will > timeout instantly after waking up without checking for the completion > (which may already be in place).
Got it. > > Instead, perform the check in the following way: > > 1. Read the current timestamp > 2. Read the completion status. If completed, return the result > 3. Sleep > 4. Check if the timestamp read at step 1 exceeds the timeout. Return > an error if it does > 5. Goto 1 > > Also, use ktime instead of jiffes as a more reliable and precise timing > source. "also", i.e. a logically separate change which should be split up to a separate patch. > > Signed-off-by: Ivan Orlov <[email protected]> > --- > drivers/char/tpm/tpm-interface.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/drivers/char/tpm/tpm-interface.c > b/drivers/char/tpm/tpm-interface.c > index 8d7e4da6ed53..959330212a16 100644 > --- a/drivers/char/tpm/tpm-interface.c > +++ b/drivers/char/tpm/tpm-interface.c > @@ -88,7 +88,8 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void > *buf, size_t bufsiz) > int rc; > ssize_t len = 0; > u32 count, ordinal; > - unsigned long stop; > + ktime_t timeout, curr_time; > + unsigned int ord_dur_us; > > if (bufsiz < TPM_HEADER_SIZE) > return -EINVAL; > @@ -126,8 +127,16 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, > void *buf, size_t bufsiz) > if (chip->flags & TPM_CHIP_FLAG_IRQ) > goto out_recv; > > - stop = jiffies + tpm_calc_ordinal_duration(chip, ordinal); > + ord_dur_us = jiffies_to_usecs(tpm_calc_ordinal_duration(chip, ordinal)); > + timeout = ktime_add_us(ktime_get(), ord_dur_us); > do { > + /* > + * Save the time of the completion check. This way even if CPU > + * goes to sleep indefinitely on tpm_sleep, the driver will > + * check for completion one more time instead of timing out > + * instantly after waking up. > + */ > + curr_time = ktime_get(); > u8 status = tpm_chip_status(chip); > if ((status & chip->ops->req_complete_mask) == > chip->ops->req_complete_val) > @@ -140,7 +149,7 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, > void *buf, size_t bufsiz) > > tpm_msleep(TPM_TIMEOUT_POLL); > rmb(); > - } while (time_before(jiffies, stop)); > + } while (ktime_before(curr_time, timeout)); Wouldn't it be simpler fix to just check completion after dropping out of the loop? } while (time_before(jiffies, stop)); if (tpm_transmit_completed(chip)) goto out_recv; And declare this before tpm_try_transmit(): static bool tpm_transmit_completed(struct tpm_chip *chip) { u8 status = tpm_chip_status(chip); return (status & chip->ops->req_complete_mask) == chip->ops->req_complete_val; } > > tpm_chip_cancel(chip); > dev_err(&chip->dev, "Operation Timed out\n"); > -- > 2.43.0 > BR, Jarkko
