Re: [Qemu-devel] [PATCH v3 for-2.11 1/3] tpm_emulator: Add a caching layer for the TPM Established flag

[Stefan Berger]

On 11/14/2017 06:40 PM, Marc-André Lureau wrote:
> Hi
>
> On Tue, Nov 14, 2017 at 10:52 PM, Stefan Berger
> <address@hidden> wrote:
> > Add a caching layer for the TPM established flag so that we don't
> > need to go to the emulator every time the flag is read by accessing
> > the REG_ACCESS register.
> What's the impact? Isn't this just a "small" optimization? Iotw, why
> is this for-2.11?

The TIS has a register that contains this flag and that's being polled 
quite frequently. So it generates a lot of traffic to the emulator. This 
caching layer gets rid of most of the traffic.

   Stefan

> > Signed-off-by: Stefan Berger <address@hidden>
> >
> > v1->v2:
> >   - move the caching to the backend layer since detecting the
> >     TPM 1.2 TSC_ResetEstablishmentBit() command is easier to do
> >     here.
> > ---
> >   hw/tpm/tpm_emulator.c | 17 ++++++++++++++---
> >   1 file changed, 14 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/tpm/tpm_emulator.c b/hw/tpm/tpm_emulator.c
> > index e1a6810..b293db7 100644
> > --- a/hw/tpm/tpm_emulator.c
> > +++ b/hw/tpm/tpm_emulator.c
> > @@ -73,6 +73,9 @@ typedef struct TPMEmulator {
> >       Error *migration_blocker;
> >
> >       QemuMutex mutex;
> > +
> > +    unsigned int established_flag:1;
> > +    unsigned int established_flag_cached:1;
> >   } TPMEmulator;
> >
> >
> > @@ -287,16 +290,22 @@ static bool 
> > tpm_emulator_get_tpm_established_flag(TPMBackend *tb)
> >       TPMEmulator *tpm_emu = TPM_EMULATOR(tb);
> >       ptm_est est;
> >
> > -    DPRINTF("%s", __func__);
> > +    if (tpm_emu->established_flag_cached) {
> > +        return tpm_emu->established_flag;
> > +    }
> > +
> >       if (tpm_emulator_ctrlcmd(tpm_emu, CMD_GET_TPMESTABLISHED, &est,
> >                                0, sizeof(est)) < 0) {
> >           error_report("tpm-emulator: Could not get the TPM established flag: 
> > %s",
> >                        strerror(errno));
> >           return false;
> >       }
> > -    DPRINTF("established flag: %0x", est.u.resp.bit);
> > +    DPRINTF("got established flag: %0x", est.u.resp.bit);
> > +
> > +    tpm_emu->established_flag_cached = 1;
> > +    tpm_emu->established_flag = (est.u.resp.bit != 0);
> >
> > -    return (est.u.resp.bit != 0);
> > +    return tpm_emu->established_flag;
> >   }
> >
> >   static int tpm_emulator_reset_tpm_established_flag(TPMBackend *tb,
> > @@ -327,6 +336,8 @@ static int 
> > tpm_emulator_reset_tpm_established_flag(TPMBackend *tb,
> >           return -1;
> >       }
> >
> > +    tpm_emu->established_flag_cached = 0;
> > +
> >       return 0;
> >   }
> >
> > --
> > 2.5.5