Re: [kvm-unit-tests PATCH 1/1] x86: use pointer for end of exception table

From: Thomas Huth <thuth@redhat.com>
To: Bill Wendling <morbo@google.com>,
	kvm@vger.kernel.org, pbonzini@redhat.com,
	alexandru.elisei@arm.com
Cc: jmattson@google.com
Subject: Re: [kvm-unit-tests PATCH 1/1] x86: use pointer for end of exception table
Date: Tue, 15 Oct 2019 09:41:26 +0200	[thread overview]
Message-ID: <6656e70c-4866-55e8-108b-9bbb2c6fd081@redhat.com> (raw)
In-Reply-To: <20191013071824.222946-1-morbo@google.com>

On 13/10/2019 09.18, Bill Wendling wrote:
> Two global objects can't have the same address in C. Clang uses this
> fact to omit the check on the first iteration of the loop in
> check_exception_table.
> 
> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
>  lib/x86/desc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/x86/desc.c b/lib/x86/desc.c
> index 451f504..cfc449f 100644
> --- a/lib/x86/desc.c
> +++ b/lib/x86/desc.c
> @@ -41,7 +41,7 @@ struct ex_record {
>      unsigned long handler;
>  };
>  
> -extern struct ex_record exception_table_start, exception_table_end;
> +extern struct ex_record exception_table_start, *exception_table_end;
>  
>  static const char* exception_mnemonic(int vector)
>  {
> @@ -113,7 +113,7 @@ static void check_exception_table(struct ex_regs *regs)
>  		(((regs->rflags >> 16) & 1) << 8);
>      asm("mov %0, %%gs:4" : : "r"(ex_val));
>  
> -    for (ex = &exception_table_start; ex != &exception_table_end; ++ex) {
> +    for (ex = &exception_table_start; ex != (void*)&exception_table_end; ++ex) {
>          if (ex->rip == regs->rip) {
>              regs->rip = ex->handler;
>              return;
> 

That looks like quite an ugly hack to me - and if clang gets a little
bit smarter, I'm pretty sure it will optimize this away, too.

Could you please check if something like this works, too:

diff --git a/lib/x86/desc.c b/lib/x86/desc.c
index 451f504..0e9779b 100644
--- a/lib/x86/desc.c
+++ b/lib/x86/desc.c
@@ -41,7 +41,8 @@ struct ex_record {
     unsigned long handler;
 };

-extern struct ex_record exception_table_start, exception_table_end;
+extern struct ex_record exception_table_start;
+extern long exception_table_size;

 static const char* exception_mnemonic(int vector)
 {
@@ -108,12 +109,13 @@ static void check_exception_table(struct ex_regs
*regs)
 {
     struct ex_record *ex;
     unsigned ex_val;
+    int cnt = exception_table_size / sizeof(struct ex_record);

     ex_val = regs->vector | (regs->error_code << 16) |
                (((regs->rflags >> 16) & 1) << 8);
     asm("mov %0, %%gs:4" : : "r"(ex_val));

-    for (ex = &exception_table_start; ex != &exception_table_end; ++ex) {
+    for (ex = &exception_table_start; cnt-- > 0; ++ex) {
         if (ex->rip == regs->rip) {
             regs->rip = ex->handler;
             return;
diff --git a/x86/flat.lds b/x86/flat.lds
index a278b56..108fad5 100644
--- a/x86/flat.lds
+++ b/x86/flat.lds
@@ -9,6 +9,7 @@ SECTIONS
           exception_table_start = .;
           *(.data.ex)
          exception_table_end = .;
+          exception_table_size = exception_table_end -
exception_table_start;
          }
     . = ALIGN(16);
     .rodata : { *(.rodata) }

 Thomas
