On Sun, Jun 22, 2025 at 09:52:58PM +0100, Jonathan McDowell wrote: > On Fri, Jun 20, 2025 at 06:08:31PM +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). > > > > Perform the completion check once again after exiting the busy loop in > > order to give the device the last chance to send us some data. > > > > Since now we check for completion in two places, extract this check into > > a separate function. > > > > Signed-off-by: Ivan Orlov <ior...@amazon.com> > > --- > > V1 -> V2: > > - Exclude the jiffies -> ktime change from the patch > > - Instead of recording the time before checking for completion, check > > for completion once again after leaving the loop > > > > drivers/char/tpm/tpm-interface.c | 17 +++++++++++++++-- > > 1 file changed, 15 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/char/tpm/tpm-interface.c > > b/drivers/char/tpm/tpm-interface.c > > index 8d7e4da6ed53..6960ee2798e1 100644 > > --- a/drivers/char/tpm/tpm-interface.c > > +++ b/drivers/char/tpm/tpm-interface.c > > @@ -82,6 +82,13 @@ static bool tpm_chip_req_canceled(struct tpm_chip *chip, > > u8 status) > > return chip->ops->req_canceled(chip, status); > > } > > > > +static bool tpm_transmit_completed(struct tpm_chip *chip) > > +{ > > + u8 status_masked = tpm_chip_status(chip) & chip->ops->req_complete_mask; > > + > > + return status_masked == chip->ops->req_complete_val; > > +} > > + > > static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t > > bufsiz) > > { > > struct tpm_header *header = buf; > > @@ -129,8 +136,7 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, > > void *buf, size_t bufsiz) > > stop = jiffies + tpm_calc_ordinal_duration(chip, ordinal); > > do { > > u8 status = tpm_chip_status(chip); > > - if ((status & chip->ops->req_complete_mask) == > > - chip->ops->req_complete_val) > > + if (tpm_transmit_completed(chip)) > > goto out_recv; > > The only thing I'd point out here is we end up doing a double status read > one after the other (once here, once in tpm_transmit_completed), and I'm > pretty sure I've seen instances where that caused a problem.
It would be easy to to prevent at least double reads after completion e.g., in tpm_chip_status(): /* * Read the chip status bitmask. After completion, the returned will mask will * return value cached at the point of completion up until the next transmit. */ static u8 tpm_chip_status(struct tpm_chip *chip) { u8 status_masked = chip->status & chip->ops_req_complete_mask; if (status_masked == chip->ops->req_complete_val) return chip->status; chip->status = tpm_chip_status(chip); return chip->status; } I think tpm_chip_status() should be the gatekeeper for such event, or like the correct layer of abstraction here ... Then just reset chip->status to zero at the beginning of tpm_try_transmit(). BR, Jarkko