Re: Direct rdtsc call side-effect

Peter Zijlstra
2023-06-01T08:56:32Z

On Thu, Jun 01, 2023 at 01:45:35PM +0500, Muhammad Usama Anjum wrote:
> Hi,
>
> I've been looking into a problem where Windows applications misbehave
> across suspend/resume when run on Wine on x86. These applications see time
> going backwards. The timestamp counter (TSC) is reset when system resumes.
> In case of Windows on Intel and AMD, the timestamp is saved and restored
> when the system resumes from suspend.
>
> These applications read timestamp by rdtsc directly. These calls cannot be
> intercepted by Wine. The application should be fixed such that it handles
> these scenarios correctly. But there are hundreds of applications which
> cannot be fixed. So some support is required in Wine or kernel. There isn't
> anything which Wine can do as rdtsc call directly reads the timestamp. The
> only option is that we support something in kernel.
>
> As more and more things are being added to Wine, Windows application can be
> run pretty easily on Linux. But this rdtsc is a big hurdle. What are your
> thoughts on solving this problem?
>
> We are thinking of saving and restoring the timestamp counter at suspend
> and resume time respectively. In theory it can work on Intel because of
> TSC_ADJUST register. But it'll never work on AMD until:
> * AMD supports the same kind of adjust register. (AMD has said that the
> adjust register cannot be implemented in their firmware. They'll have to
> add it to their hardware.)
> * by manual synchronization in kernel (I know you don't like this idea. But
> there is something Windows is doing to save/restore and sync the TSC)

Wine could set TIF_NOTSC, which will cause it to run with CR4.TSD
cleared and cause RDTSC to #GP, at which point you can emulate it.