Re: [PATCH v4] x86/power: Fix 'nosmt' vs. hibernation triple fault during resume

[Josh Poimboeuf]

On Fri, May 31, 2019 at 01:42:02AM +0200, Jiri Kosina wrote:
> On Thu, 30 May 2019, Josh Poimboeuf wrote:
> 
> > > >     Reviewed-by: Thomas Gleixner <t...@linutronix.de>
> > > 
> > > Yes, it is, thanks!
> > 
> > I still think changing monitor/mwait to use a fixmap address would be a
> > much cleaner way to fix this.  I can try to work up a patch tomorrow.
> 
> I disagree with that from the backwards compatibility point of view.
> 
> I personally am quite frequently using differnet combinations of 
> resumer/resumee kernels, and I've never been biten by it so far. I'd guess 
> I am not the only one.
> Fixmap sort of breaks that invariant.

Right now there is no backwards compatibility because nosmt resume is
already broken.

For "future" backwards compatibility we could just define a hard-coded
reserved fixmap page address, adjacent to the vsyscall reserved address.

Something like this (not yet tested)?  Maybe we could also remove the
resume_play_dead() hack?

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index 9da8cccdf3fb..1c328624162c 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -80,6 +80,7 @@ enum fixed_addresses {
 #ifdef CONFIG_X86_VSYSCALL_EMULATION
        VSYSCALL_PAGE = (FIXADDR_TOP - VSYSCALL_ADDR) >> PAGE_SHIFT,
 #endif
+       FIX_MWAIT = (FIXADDR_TOP - VSYSCALL_ADDR - 1) >> PAGE_SHIFT,
 #endif
        FIX_DBGP_BASE,
        FIX_EARLYCON_MEM_BASE,
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 73e69aaaa117..9804fbe25d03 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -108,6 +108,8 @@ int __read_mostly __max_smt_threads = 1;
 /* Flag to indicate if a complete sched domain rebuild is required */
 bool x86_topology_update;
 
+static char __mwait_page[PAGE_SIZE];
+
 int arch_update_cpu_topology(void)
 {
        int retval = x86_topology_update;
@@ -1319,6 +1321,8 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
        smp_quirk_init_udelay();
 
        speculative_store_bypass_ht_init();
+
+       set_fixmap(FIX_MWAIT, __pa_symbol(&__mwait_page));
 }
 
 void arch_enable_nonboot_cpus_begin(void)
@@ -1631,11 +1635,12 @@ static inline void mwait_play_dead(void)
        }
 
        /*
-        * This should be a memory location in a cache line which is
-        * unlikely to be touched by other processors.  The actual
-        * content is immaterial as it is not actually modified in any way.
+        * This memory location is never actually written to.  It's mapped at a
+        * reserved fixmap address to ensure the monitored address remains
+        * valid across a hibernation resume operation.  Otherwise a triple
+        * fault can occur.
         */
-       mwait_ptr = &current_thread_info()->flags;
+       mwait_ptr = (void *)fix_to_virt(FIX_MWAIT);
 
        wbinvd();