Messages in this thread Patch in this message | | | From | Thomas Gleixner <> | Subject | Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions | Date | Thu, 23 Jul 2020 21:53:20 +0200 |
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Ira,
ira.weiny@intel.com writes:
> ... > // ref == 0 > dev_access_enable() // ref += 1 ==> disable protection > irq() > // enable protection > // ref = 0 > _handler() > dev_access_enable() // ref += 1 ==> disable protection > dev_access_disable() // ref -= 1 ==> enable protection > // WARN_ON(ref != 0) > // disable protection > do_pmem_thing() // all good here > dev_access_disable() // ref -= 1 ==> 0 ==> enable protection
...
> First I'm not sure if adding this state to idtentry_state and having > that state copied is the right way to go.
Adding the state to idtentry_state is fine at least for most interrupts and exceptions. Emphasis on most.
#PF does not work because #PF can schedule.
> It seems like we should start passing this by reference instead of > value. But for now this works as an RFC. Comments?
Works as in compiles, right?
static void noinstr idt_save_pkrs(idtentry_state_t state) { state.foo = 1; }
How is that supposed to change the caller state? C programming basics.
> Second, I'm not 100% happy with having to save the reference count in > the exception handler. It seems like a very ugly layering violation but > I don't see a way around it at the moment.
That state is strict per task, right? So why do you want to store it anywhere else that in task/thread storage. That solves your problem of #PF scheduling nicely.
> Third, this patch has gone through a couple of revisions as I've had > crashes which just don't make sense to me. One particular issue I've > had is taking a MCE during memcpy_mcsafe causing my WARN_ON() to fire. > The code path was a pmem copy and the ref count should have been > elevated due to dev_access_enable() but why was > idtentry_enter()->idt_save_pkrs() not called I don't know.
Because #MC does not go through idtentry_enter(). Neither do #NMI, #DB, #BP.
> Finally, it looks like the entry/exit code is being refactored into > common code. So likely this is best handled somewhat there. Because > this can be predicated on CONFIG_ARCH_HAS_SUPERVISOR_PKEYS and handled > in a generic fashion. But that is a ways off I think.
The invocation of save/restore might be placed in generic code at least for the common exception and interrupt entries.
> +static void noinstr idt_save_pkrs(idtentry_state_t state)
*state. See above.
> +#else > +/* Define as macros to prevent conflict of inline and noinstr */ > +#define idt_save_pkrs(state) > +#define idt_restore_pkrs(state)
Empty inlines do not need noinstr because they are optimized out. If you want inlines in a noinstr section then use __always_inline
> /** > * idtentry_enter - Handle state tracking on ordinary idtentries > * @regs: Pointer to pt_regs of interrupted context > @@ -604,6 +671,8 @@ idtentry_state_t noinstr idtentry_enter(struct pt_regs *regs) > return ret; > } > > + idt_save_pkrs(ret);
No. This really has no business to be called before the state is established. It's not something horribly urgent and write_pkrs() is NOT noinstr and invokes wrmsrl() which is subject to tracing.
> + > + idt_restore_pkrs(state);
This one is placed correctly.
Thanks,
tglx
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