Re: [Qemu-devel] [PATCH] kvm-unittest: fix build with gcc 4.3.X and older

[Michael S. Tsirkin]

On Thu, Oct 17, 2013 at 09:27:51AM +0300, Gleb Natapov wrote:
> On Wed, Oct 16, 2013 at 10:46:53PM +0300, Michael S. Tsirkin wrote:
> > Old GCC didn't let you reference variable by
> > number if it is listed with a specific register
> > constraint, on the assumption you can just
> > use the register name explicitly.
> > 
> > Build fails with errors like this:
> > a.c:6: error: invalid 'asm': invalid operand code 'd'
> > 
> Is it worth to support such ancient compiler? Nobody complained till
> now.

Well it's not as widely used as kvm itself yet :)
The patch seems simple enough though.

> BTW with your patch I still cannot compile with 4.2:
> 
> x86/s3.c: In function 'main':
> x86/s3.c:145: error: inconsistent operand constraints in an 'asm'

OK that's easy to fix.

> > To fix, let's just use %eax %al etc.
> > 
> Only %d0 does not work and dropping "d" fixes it since compiler can
> figure out correct register from variable size. The patch bellow fixes
> compilation for 4.2.

It does produce warnings with -Wall though:
Assembler messages:
Warning: using `%ax' instead of `%eax' due to `w' suffix
Warning: using `%al' instead of `%eax' due to `b' suffix

So it's not the compiler, the compiler generated wrong
code with outb %eax. But assembler saw outb and %eax which
are inconsistent and decided that outb should take precedence.

I'm not sure it's wize to rely on this behaviour though.
How's this? This should fix 4.2 for you too.

Maybe we should tweak outb/outw as well to keep it
all consistent. I can do this - what do you think?


diff --git a/lib/x86/pci.c b/lib/x86/pci.c
index f95cd88..08f7ebf 100644
--- a/lib/x86/pci.c
+++ b/lib/x86/pci.c
@@ -3,13 +3,13 @@
 
 static void outl(unsigned short port, unsigned val)
 {
-    asm volatile("outl %d0, %w1" : : "a"(val), "Nd"(port));
+    asm volatile("outl %%eax, %w1" : : "a"(val), "Nd"(port));
 }
 
 static unsigned inl(unsigned short port)
 {
     unsigned data;
-    asm volatile("inl %w1, %d0" : "=a"(data) : "Nd"(port));
+    asm volatile("inl %w1, %%eax" : "=a"(data) : "Nd"(port));
     return data;
 }
 static uint32_t pci_config_read(pcidevaddr_t dev, uint8_t reg)
diff --git a/x86/s3.c b/x86/s3.c
index 1feb452..bfa4206 100644
--- a/x86/s3.c
+++ b/x86/s3.c
@@ -142,15 +142,15 @@ u32* find_resume_vector_addr(void)
 static inline int rtc_in(u8 reg)
 {
     u8 x = reg;
-    asm volatile("outb %b1, $0x70; inb $0x71, %b0"
-                : "+a"(x) : "0"(x));
+    asm volatile("outb %%al, $0x70; inb $0x71, %%al"
+                : "+a"(x));
     return x;
 }
 
 static inline void rtc_out(u8 reg, u8 val)
 {
-    asm volatile("outb %b1, $0x70; mov %b2, %b1; outb %b1, $0x71"
-                : "+a"(reg) : "0"(reg), "ri"(val));
+    asm volatile("outb %%al, $0x70; mov %b1, %%al; outb %%al, $0x71"
+                : "+a"(reg) : "ri"(val));
 }
 
 extern char resume_start, resume_end;
diff --git a/x86/vmexit.c b/x86/vmexit.c
index 3b945de..29bc582 100644
--- a/x86/vmexit.c
+++ b/x86/vmexit.c
@@ -26,7 +26,7 @@ static void outw(unsigned short port, unsigned val)
 
 static void outl(unsigned short port, unsigned val)
 {
-    asm volatile("outl %d0, %w1" : : "a"(val), "Nd"(port));
+    asm volatile("outl %%eax, %w1" : : "a"(val), "Nd"(port));
 }
 
 static unsigned int inb(unsigned short port)


-- 
MST