Re: [RFC PATCH glibc 11/12] hurd, htl: Add some x86_64-specific code

[Samuel Thibault]

Sergey Bugaev, le dim. 12 févr. 2023 19:51:33 +0300, a ecrit:
> On Sun, Feb 12, 2023 at 7:36 PM Samuel Thibault <samuel.thibault@gnu.org> 
> wrote:
> >
> > Sergey Bugaev, le dim. 12 févr. 2023 19:25:11 +0300, a ecrit:
> > > On Sun, Feb 12, 2023 at 7:11 PM Samuel Thibault <samuel.thibault@gnu.org> 
> > > wrote:
> > > > Sergey Bugaev, le dim. 12 févr. 2023 14:10:42 +0300, a ecrit:
> > > > > We should not need a getter routine, because one can simply inspect 
> > > > > the target
> > > > > thread's state (unless, again, I misunderstand things horribly).
> > > >
> > > > For 16bit fs/gs values we could read them from userland yes. But for
> > > > fs/gs base, the FSGSBASE instruction is not available on all 64bit
> > > > processors. And ATM in THREAD_TCB we want to be able to get the base of
> > > > another thread.
> > >
> > > What I've meant is:
> > >
> > > __thread_get_state (whatever_thread, &state);
> > > uintptr_t its_fs_base = state->fs_base;
> > >
> > > You can't really do the same to *write* [fg]s_base, because doing
> > > thread_set_state on your own thread is bound to end badly.
> >
> > ? Well, sure, just like setting fs/gs through thread state was not done
> > for i386.
> >
> > I don't see where you're aiming. Getting fs/gs from __thread_get_state
> > won't actually give you the base, you'll just read something like 0.
> 
> It is my understanding that the actual values of fs/gs (i.e. the index
> of a descriptor) are not useful on x86_64. But fs_base and gs_base are
> now things that you have to store in the thread state and save/restore
> on every context switch. fs_base and gs_base are like registers in
> their own right (well, MSRs are registers). Thus, it should be easy to
> read them from the state structure exposed by the kernel.

Ah, so you mean adding the fs/gs bases to the thread_state content.

I'd rather make it a separate state content, like we have a separate
i386_DEBUG_STATE content.

(And thus no need for a new RPC).

Samuel