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Re: [PATCH v2] goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/T

From: Richard Henderson
Subject: Re: [PATCH v2] goldfish_rtc: Fix non-atomic read behaviour of TIME_LOW/TIME_HIGH
Date: Sat, 18 Jul 2020 11:56:50 -0700
User-agent: 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
> read.
> Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
> ---
> 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 <richard.henderson@linaro.org>


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