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Re: [PATCH v2] goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/T
Re: [PATCH v2] goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/TIME_HIGH
Sat, 18 Jul 2020 11:56:50 -0700
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0
On 7/17/20 5:49 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
> Signed-off-by: Jessica Clarke <firstname.lastname@example.org>
> Changes since v1:
> * Add time_high to goldfish_rtc_vmstate and increment version.
> hw/rtc/goldfish_rtc.c | 17 ++++++++++++++---
> include/hw/rtc/goldfish_rtc.h | 1 +
> 2 files changed, 15 insertions(+), 3 deletions(-)
Reviewed-by: Richard Henderson <email@example.com>