Re: [PATCH v2] tcg: Fix execution on Apple Silicon

[Roman Bolshakov]

On Mon, Jan 04, 2021 at 07:39:13PM +0100, Alexander Graf wrote:
> 
> On 04.01.21 16:23, Alex Bennée wrote:
> > Roman Bolshakov <r.bolshakov@yadro.com> writes:
> > 
> > > Pages can't be both write and executable at the same time on Apple
> > > Silicon. macOS provides public API to switch write protection [1] for
> > > JIT applications, like TCG.
> > > 
> > > 1. 
> > > https://developer.apple.com/documentation/apple_silicon/porting_just-in-time_compilers_to_apple_silicon
> > > 
> > > Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
> > > ---
> > > v1: https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg00073.html
> > > Changes since v1:
> > > 
> > >   - Pruned not needed fiddling with W^X and dropped symmetry from write
> > >     lock/unlock and renamed related functions.
> > >     Similar approach is used in JavaScriptCore [1].
> > > 
> > >   - Moved jit helper functions to util/osdep
> > >                                                                           
> > >                                                                          
> > > As outlined in osdep.h, this matches to (2):                              
> > >                                                                           
> > >                                                                           
> > >                                                                   * In an 
> > > ideal world this header would contain only:                               
> > >                                                              *  (1) 
> > > things which everybody needs                                              
> > >                                                               *  (2) 
> > > things without which code would work on most platforms but                
> > >                                                               *      fail 
> > > to compile or misbehave on a minority of host OSes
> > > 
> > >   - Fixed a checkpatch error
> > > 
> > >   - Limit new behaviour only to macOS 11.0 and above, because of the
> > >     following declarations:
> > > 
> > >     __API_AVAILABLE(macos(11.0))
> > >     __API_UNAVAILABLE(ios, tvos, watchos)
> > >     void pthread_jit_write_protect_np(int enabled);
> > > 
> > >     __API_AVAILABLE(macos(11.0))
> > >     __API_UNAVAILABLE(ios, tvos, watchos)
> > >     int pthread_jit_write_protect_supported_np(void);
> > > 
> > >   1. https://bugs.webkit.org/attachment.cgi?id=402515&action=prettypatch
> > > 
> > >   accel/tcg/cpu-exec.c      |  2 ++
> > >   accel/tcg/translate-all.c |  6 ++++++
> > >   include/qemu/osdep.h      |  3 +++
> > >   tcg/tcg.c                 |  1 +
> > >   util/osdep.c              | 22 ++++++++++++++++++++++
> > >   5 files changed, 34 insertions(+)
> > > 
> > > diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> > > index 8689c54499..374060eb45 100644
> > > --- a/accel/tcg/cpu-exec.c
> > > +++ b/accel/tcg/cpu-exec.c
> > > @@ -175,6 +175,7 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState 
> > > *cpu, TranslationBlock *itb)
> > >       }
> > >   #endif /* DEBUG_DISAS */
> > > +    qemu_thread_jit_execute();
> > >       ret = tcg_qemu_tb_exec(env, tb_ptr);
> > >       cpu->can_do_io = 1;
> > >       last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
> > > @@ -382,6 +383,7 @@ static inline void tb_add_jump(TranslationBlock *tb, 
> > > int n,
> > >   {
> > >       uintptr_t old;
> > > +    qemu_thread_jit_write();
> > >       assert(n < ARRAY_SIZE(tb->jmp_list_next));
> > >       qemu_spin_lock(&tb_next->jmp_lock);
> > > diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> > > index b7d50a73d4..88ae5d35ef 100644
> > > --- a/accel/tcg/translate-all.c
> > > +++ b/accel/tcg/translate-all.c
> > > @@ -1072,6 +1072,9 @@ static inline void *alloc_code_gen_buffer(void)
> > >       size_t size = tcg_ctx->code_gen_buffer_size;
> > >       void *buf;
> > > +#if defined(__APPLE__) && defined(MAC_OS_VERSION_11_0)
> > > +    flags |= MAP_JIT;
> > > +#endif
> > >       buf = mmap(NULL, size, prot, flags, -1, 0);
> > >       if (buf == MAP_FAILED) {
> > >           return NULL;
> > > @@ -1485,7 +1488,9 @@ static void do_tb_phys_invalidate(TranslationBlock 
> > > *tb, bool rm_from_page_list)
> > >   static void tb_phys_invalidate__locked(TranslationBlock *tb)
> > >   {
> > > +    qemu_thread_jit_write();
> > >       do_tb_phys_invalidate(tb, true);
> > > +    qemu_thread_jit_execute();
> > >   }
> > >   /* invalidate one TB
> > > @@ -1687,6 +1692,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
> > >   #endif
> > >       assert_memory_lock();
> > > +    qemu_thread_jit_write();
> > >       phys_pc = get_page_addr_code(env, pc);
> > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index f9ec8c84e9..89abebcf5d 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -686,4 +686,7 @@ char *qemu_get_host_name(Error **errp);
> > >    */
> > >   size_t qemu_get_host_physmem(void);
> > > +void qemu_thread_jit_write(void);
> > > +void qemu_thread_jit_execute(void);
> > > +
> > >   #endif
> > > diff --git a/tcg/tcg.c b/tcg/tcg.c
> > > index 43c6cf8f52..ab8488f5d5 100644
> > > --- a/tcg/tcg.c
> > > +++ b/tcg/tcg.c
> > > @@ -1065,6 +1065,7 @@ void tcg_prologue_init(TCGContext *s)
> > >       s->pool_labels = NULL;
> > >   #endif
> > > +    qemu_thread_jit_write();
> > >       /* Generate the prologue.  */
> > >       tcg_target_qemu_prologue(s);
> > > diff --git a/util/osdep.c b/util/osdep.c
> > > index 66d01b9160..80ec7185da 100644
> > > --- a/util/osdep.c
> > > +++ b/util/osdep.c
> > > @@ -606,3 +606,25 @@ writev(int fd, const struct iovec *iov, int iov_cnt)
> > >       return readv_writev(fd, iov, iov_cnt, true);
> > >   }
> > >   #endif
> > > +
> > > +#if defined(__APPLE__) && defined(MAC_OS_VERSION_11_0)
> 
> 
> Will this be defined in future versions?
> 

Yes, it will be. But we might explicitly specify upper and lower bounds:

https://github.com/phracker/MacOSX-SDKs/blob/master/MacOSX10.15.sdk/usr/include/AvailabilityMacros.h

> 
> > > +static inline void qemu_thread_jit_write_protect(bool enabled)
> > > +{
> > > +    if (pthread_jit_write_protect_supported_np()) {
> 
> 
> Do we need this call? Sounds like extra overhead to me.
> 

We don't need it on ARM, but Apple uses it for x86_64 in JavaScriptCore:

https://bugs.webkit.org/attachment.cgi?id=402515&action=prettypatch

> 
> > > +        pthread_jit_write_protect_np(enabled);
> > > +    }
> > > +}
> > > +
> > > +void qemu_thread_jit_execute(void)
> > > +{
> > > +    qemu_thread_jit_write_protect(true);
> > > +}
> > > +
> > > +void qemu_thread_jit_write(void)
> > > +{
> > > +    qemu_thread_jit_write_protect(false);
> > > +}
> > What happens if you emulate a -smp 2 ARM guest? In this case MTTCG
> > should be enabled (same guest ordering) but you run a risk of attempting
> > to execute code while write is enabled.
> > 
> > Is there any way to only change the mapping for the parts of the TB
> > cache used by a thread? Otherwise we'll need additional logic in
> > default_mttcg_enabled to ensure we don't accidentally enable it on Apple
> > silicon.
> 
> 
> The actual protection logic is per thread, so the MTTCG side thread won't be
> affected by the flips.
> 

That's correct.

> Given this super specific semantic that is impossible to mimic on other
> platforms, we should probably name the functions accordingly and make sure
> people understand this is *only* for macos.
> 

I intended to name them accordingly :) I'm sorry if it's not clear. What
do you propose instead?

> Also, is there anything this patch doesn't do that the one from Joelle does?
> It seems a bit ... short.
> 

I tried to minimize amount of W^X fiddling to the bare minimum required
for correct operation of TCG. So if you remove any of
qemu_thread_jit_*() calls in the patch, TCG won't work.

Thanks,
Roman