Re: Direct rdtsc call side-effect

Peter Zijlstra
2023-06-01T08:59:09Z

On Thu, Jun 01, 2023 at 10:56:03AM +0200, Peter Zijlstra wrote:
> 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.

The other option is to have Wine run itself in a (KVM) virtual machine
and mess with the VMM TSC offset :-)