| Message ID | 20200718002027.82300-1-jrtc27@jrtc27.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/TIME_HIGH | expand |
On 7/17/20 5:20 PM, Jessica Clarke wrote: > The specification says: > > 0x00 TIME_LOW R: Get current time, then return low-order 32-bits. > 0x04 TIME_HIGH R: Return high 32-bits from previous TIME_LOW read. > > ... > > To read the value, the kernel must perform an IO_READ(TIME_LOW), > which returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH), > which returns a signed 32-bit value, corresponding to the higher half > of the full value. > > However, we were just returning the current time for both. If the guest > is unlucky enough to read TIME_LOW and TIME_HIGH either side of an > overflow of the lower half, it will see time be in the future, before > jumping backwards on the next read, and Linux currently relies on the > atomicity guaranteed by the spec so is affected by this. Fix this > violation of the spec by caching the correct value for TIME_HIGH > whenever TIME_LOW is read, and returning that value for any TIME_HIGH > read. > > Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com> > --- > hw/rtc/goldfish_rtc.c | 14 ++++++++++++-- > include/hw/rtc/goldfish_rtc.h | 1 + > 2 files changed, 13 insertions(+), 2 deletions(-) > > diff --git a/hw/rtc/goldfish_rtc.c b/hw/rtc/goldfish_rtc.c > index 01e9d2b083..9b577bf159 100644 > --- a/hw/rtc/goldfish_rtc.c > +++ b/hw/rtc/goldfish_rtc.c > @@ -94,12 +94,22 @@ static uint64_t goldfish_rtc_read(void *opaque, hwaddr offset, > GoldfishRTCState *s = opaque; > uint64_t r = 0; > > + /* > + * From the documentation linked at the top of the file: > + * > + * To read the value, the kernel must perform an IO_READ(TIME_LOW), which > + * returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH), which > + * returns a signed 32-bit value, corresponding to the higher half of the > + * full value. > + */ > switch (offset) { > case RTC_TIME_LOW: > - r = goldfish_rtc_get_count(s) & 0xffffffff; > + r = goldfish_rtc_get_count(s); > + s->time_high = r >> 32; > + r &= 0xffffffff; > break; > case RTC_TIME_HIGH: > - r = goldfish_rtc_get_count(s) >> 32; > + r = s->time_high; > break; > case RTC_ALARM_LOW: > r = s->alarm_next & 0xffffffff; > diff --git a/include/hw/rtc/goldfish_rtc.h b/include/hw/rtc/goldfish_rtc.h > index 16f9f9e29d..9bd8924f5f 100644 > --- a/include/hw/rtc/goldfish_rtc.h > +++ b/include/hw/rtc/goldfish_rtc.h > @@ -41,6 +41,7 @@ typedef struct GoldfishRTCState { > uint32_t alarm_running; > uint32_t irq_pending; > uint32_t irq_enabled; > + uint32_t time_high; > } GoldfishRTCState; You need to add the new field to goldfish_rtc_vmstate, and increment the version. r~
diff --git a/hw/rtc/goldfish_rtc.c b/hw/rtc/goldfish_rtc.c index 01e9d2b083..9b577bf159 100644 --- a/hw/rtc/goldfish_rtc.c +++ b/hw/rtc/goldfish_rtc.c @@ -94,12 +94,22 @@ static uint64_t goldfish_rtc_read(void *opaque, hwaddr offset, GoldfishRTCState *s = opaque; uint64_t r = 0; + /* + * From the documentation linked at the top of the file: + * + * To read the value, the kernel must perform an IO_READ(TIME_LOW), which + * returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH), which + * returns a signed 32-bit value, corresponding to the higher half of the + * full value. + */ switch (offset) { case RTC_TIME_LOW: - r = goldfish_rtc_get_count(s) & 0xffffffff; + r = goldfish_rtc_get_count(s); + s->time_high = r >> 32; + r &= 0xffffffff; break; case RTC_TIME_HIGH: - r = goldfish_rtc_get_count(s) >> 32; + r = s->time_high; break; case RTC_ALARM_LOW: r = s->alarm_next & 0xffffffff; diff --git a/include/hw/rtc/goldfish_rtc.h b/include/hw/rtc/goldfish_rtc.h index 16f9f9e29d..9bd8924f5f 100644 --- a/include/hw/rtc/goldfish_rtc.h +++ b/include/hw/rtc/goldfish_rtc.h @@ -41,6 +41,7 @@ typedef struct GoldfishRTCState { uint32_t alarm_running; uint32_t irq_pending; uint32_t irq_enabled; + uint32_t time_high; } GoldfishRTCState; #endif
The specification says: 0x00 TIME_LOW R: Get current time, then return low-order 32-bits. 0x04 TIME_HIGH R: Return high 32-bits from previous TIME_LOW read. ... To read the value, the kernel must perform an IO_READ(TIME_LOW), which returns an unsigned 32-bit value, before an IO_READ(TIME_HIGH), which returns a signed 32-bit value, corresponding to the higher half of the full value. However, we were just returning the current time for both. If the guest is unlucky enough to read TIME_LOW and TIME_HIGH either side of an overflow of the lower half, it will see time be in the future, before jumping backwards on the next read, and Linux currently relies on the atomicity guaranteed by the spec so is affected by this. Fix this violation of the spec by caching the correct value for TIME_HIGH whenever TIME_LOW is read, and returning that value for any TIME_HIGH read. Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com> --- hw/rtc/goldfish_rtc.c | 14 ++++++++++++-- include/hw/rtc/goldfish_rtc.h | 1 + 2 files changed, 13 insertions(+), 2 deletions(-)