Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 12 Nov 2019 09:41:58 -0800 | Subject | Re: [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to user work |
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On Tue, Nov 12, 2019 at 9:20 AM Thomas Gleixner <tglx@linutronix.de> wrote: > > On Tue, 12 Nov 2019, Andy Lutomirski wrote: > > On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > > > > > There is no point to update the TSS bitmap for tasks which use I/O bitmaps > > > on every context switch. It's enough to update it right before exiting to > > > user space. > > + > > > +static inline void switch_to_bitmap(unsigned long tifp) > > > +{ > > > + /* > > > + * Invalidate I/O bitmap if the previous task used it. If the next > > > + * task has an I/O bitmap it will handle it on exit to user mode. > > > + */ > > > + if (tifp & _TIF_IO_BITMAP) > > > + tss_invalidate_io_bitmap(this_cpu_ptr(&cpu_tss_rw)); > > > +} > > > > Shouldn't you be invalidating the io bitmap if the *next* task doesn't > > use? Or is the rule that, when a non-io-bitmap-using task is running, > > even in kernel mode, the io bitmap is always invalid. > > Well it does not make much of a difference whether we do the above or > !(tifn & _TIF_IO_BITMAP). We always end up in that code when one of the > involved tasks has TIF_IO_BITMAP set. I decided to use the sched out check > because that makes it clear that this is the end of the valid I/O > bitmap. If the next task has TIF_IO_BITMAP set as well, then it will anyway > end up in the exit to user mode update code. Clearing it here ensures that > even if the exit to user mode malfunctions the bitmap cannot be leaked. > > > As it stands, you need exit_thread() to invalidate the bitmap. I > > assume it does, but I can't easily see it in the middle of the series > > like this. > > It does. > > > IOW your code might be fine, but it could at least use some comments > > in appropriate places (exit_to_usermode_loop()?) that we guarantee > > that, if the bit is *clear*, then the TSS has the io bitmap marked > > invalid. And add an assertion under CONFIG_DEBUG_ENTRY. > > > > Also, do you need to update EXIT_TO_USERMODE_LOOP_FLAGS? > > No, the TIF_IO_BITMAP check is done once after the loop has run and it > would not make any sense in the loop as TIF_IO_BITMAP cannot be cleared > there and we'd loop forever. The other usermode loop flags are transient. >
Right. But your diff tool *said* the diff was in exit_to_usermode_loop(). Can you look at your .gitconfig and see if you have something weird going on?
--Andy
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