Re: [regression] significant delays when secureboot is enabled since 6.10

Jarkko Sakkinen
2024-09-12T14:26:18Z

On Thu Sep 12, 2024 at 4:26 PM EEST, James Bottomley wrote:
> On Thu, 2024-09-12 at 16:16 +0300, Jarkko Sakkinen wrote:
> > On Wed Sep 11, 2024 at 3:21 PM EEST, James Bottomley wrote:
> > > On Wed, 2024-09-11 at 10:53 +0200, Roberto Sassu wrote:
> [...]
> > > > I made few measurements. I have a Fedora 38 VM with TPM
> > > > passthrough.
> > > >
> > > > Kernels: 6.11-rc2+ (guest), 6.5.0-45-generic (host)
> > > >
> > > > QEMU:
> > > >
> > > > rc  qemu-kvm                                          1:4.2-
> > > > 3ubuntu6.27
> > > > ii  qemu-system-x86                                   1:6.2+dfsg-
> > > > 2ubuntu6.22
> > > >
> > > >
> > > > TPM2_PT_MANUFACTURER:
> > > >   raw: 0x49465800
> > > >   value: "IFX"
> > > > TPM2_PT_VENDOR_STRING_1:
> > > >   raw: 0x534C4239
> > > >   value: "SLB9"
> > > > TPM2_PT_VENDOR_STRING_2:
> > > >   raw: 0x36373000
> > > >   value: "670"
> > > >
> > > >
> > > > No HMAC:
> > > >
> > > > # tracer: function_graph
> > > > #
> > > > # CPU  DURATION                  FUNCTION CALLS
> > > > # |     |   |                     |   |   |   |
> > > >  0)               |  tpm2_pcr_extend() {
> > > >  0)   1.112 us    |    tpm_buf_append_hmac_session();
> > > >  0) # 6360.029 us |    tpm_transmit_cmd();
> > > >  0) # 6415.012 us |  }
> > > >
> > > >
> > > > HMAC:
> > > >
> > > > # tracer: function_graph
> > > > #
> > > > # CPU  DURATION                  FUNCTION CALLS
> > > > # |     |   |                     |   |   |   |
> > > >  1)               |  tpm2_pcr_extend() {
> > > >  1)               |    tpm2_start_auth_session() {
> > > >  1) * 36976.99 us |      tpm_transmit_cmd();
> > > >  1) * 84746.51 us |      tpm_transmit_cmd();
> > > >  1) # 3195.083 us |      tpm_transmit_cmd();
> > > >  1) @ 126795.1 us |    }
> > > >  1)   2.254 us    |    tpm_buf_append_hmac_session();
> > > >  1)   3.546 us    |    tpm_buf_fill_hmac_session();
> > > >  1) * 24356.46 us |    tpm_transmit_cmd();
> > > >  1)   3.496 us    |    tpm_buf_check_hmac_response();
> > > >  1) @ 151171.0 us |  }
> > >
> > > Well, unfortunately, that tells us that it's the TPM itself that's
> > > taking the time processing the security overhead.  The ordering of
> > > the commands in tpm2_start_auth_session() shows
> > >
> > >  37ms for context restore of null key
> > >  85ms for start session with encrypted salt
> > >   3ms to flush null key
> > > -----
> > > 125ms
> > >
> > > If we context save the session, we'd likely only bear a single 37ms
> > > cost to restore it (replacing the total 125ms).  However, there's
> > > nothing we can do about the extend execution going from 6ms to
> > > 24ms, so I could halve your current boot time with security enabled
> > > (it's currently 149ms, it would go to 61ms, but it's still 10x
> > > slower than the unsecured extend at 6ms)
> > >
> > > James
> >
> > I'll hold for better benchmarks.
>
> Well, yes, I'd like to see this for a variety of TPMs.
>
> This one clearly shows it's the real time wait for the TPM (since it
> dwarfs the CPU time calculation there's not much optimization we can do
> on the kernel end). The one thing that's missing in all of this is
> what was the TPM? but even if it's an outlier that's really bad at
> crypto what should we do? We could have a blacklist that turns off the
> extend hmac (or a whitelist that turns it on), but we can't simply say
> too bad you need a better TPM.
>
> James

I'm pasting here my yesterday's one-liner ;-)

sudo bpftrace -e 'k:tpm_transmit { @start[tid] = nsecs; } kr:tpm_transmit { @[kstack, ustack, comm] = sum(nsecs - @start[tid]); delete(@start[tid]); } END { clear(@start); }'

If you have a fix candidate, snippet of the output before/after would
work as rationale too.

Looking into the data Roberto put me tomorrow.

BR, Jarkko