Messages in this thread | | | Date | Wed, 03 Oct 2007 12:25:54 +0400 | From | Kirill Korotaev <> | Subject | Re: [PATCH] mark read_crX() asm code as volatile |
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Arjan,
I can experiment with any constraints if you suggest which one.
From our experiments with gcc, it compares asm strings (sic!!!) to find matches to be merged! Sigh... Below are 2 programs which differ in one space in read_cr3_b() asm statement. The first one compiles incorrectly, while 2nd one - correctly.
My personal feeling is that comparing asm strings is simply a "misfeature".
-------------------------- cut ---------------------------- static inline unsigned long read_cr3_a(void) { unsigned long cr3; asm("movq %%cr3,%0" : "=r" (cr3)); return cr3; }
static inline unsigned long read_cr3_b(void) { unsigned long cr3; asm("movq %%cr3,%0" : "=r" (cr3)); return cr3; }
static inline void write_cr3(unsigned long val) { asm volatile("movq %0,%%cr3" :: "r" (val) : "memory"); }
void main() { unsigned long c; c = read_cr3_a(); write_cr3(c | 0x80); c = read_cr3_b(); write_cr3(c | 0x100); } -------------------------- cut ----------------------------
-------------------------- cut ---------------------------- static inline unsigned long read_cr3_a(void) { unsigned long cr3; asm("movq %%cr3,%0" : "=r" (cr3)); return cr3; }
static inline unsigned long read_cr3_b(void) { unsigned long cr3; asm("movq %%cr3,%0" : "=r" (cr3)); return cr3; }
static inline void write_cr3(unsigned long val) { asm volatile("movq %0,%%cr3" :: "r" (val) : "memory"); }
void main() { unsigned long c; c = read_cr3_a(); write_cr3(c | 0x80); c = read_cr3_b(); write_cr3(c | 0x100); } -------------------------- cut ----------------------------
Kirill
Arjan van de Ven wrote: > On Tue, 02 Oct 2007 18:08:32 +0400 > Kirill Korotaev <dev@openvz.org> wrote: > > >>Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from >>gentoo) can generate incorrect code with read_crX()/write_crX() >>functions mix up, due to cached results of read_crX(). >> > > > I'm not so sure volatile is the right answer, as compared to giving the > asm more strict contraints.... > > asm volatile tends to mean something else than "the result has > changed".... > >
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