Re: Direct rdtsc call side-effect

Thomas Gleixner
2023-06-01T18:20:53Z

On Thu, Jun 01 2023 at 12:26, Thomas Gleixner wrote:
> On Thu, Jun 01 2023 at 13:45, Muhammad Usama Anjum wrote:
>> We are thinking of saving and restoring the timestamp counter at suspend
>> and resume time respectively.

I assume you talk about suspend to disk here, right? Suspend to RAM
definitely does not have the problem at least not on any halfways
contemporary CPU.

>> 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)
>
> Synchronizing TSC by writing the TSC MSR is fragile as hell. This has
> been tried so often and never reliably passed all synchronization tests
> on a wide range of systems.
>
> It kinda works on single socket, but not on larger systems.

Here is an example where it falls flat on its nose.

One of the early Ryzen laptops had a broken BIOS which came up with
unsynchronized TSCs. I tried to fix that up, but couldn't get it to sync
on all CPUs because for some stupid reason the TSC write got
arbritrarily delayed (assumably by SMI/SMM).

After the vendor fixed the BIOS, I tried again and the problem
persisted.

So on such a machine the 'fixup time' mechanism would simply render an
otherwise perfectly fine TSC unusable for timekeeping.

We asked both Intel and AMD to add TSC_ADJUST probably 15 years
ago. Intel added it with some HSW variants (IIRC) and since SKL all CPUs
have it. I don't know why AMD thought it's not required. That could have
spared a gazillion of bugzilla entries vs. the early Ryzen machines.

Thanks,

tglx