Messages in this thread | | | Subject | Re: [PATCH v5 1/5] x86/asm: Carve out a generic movdir64b() helper for general usage | From | Dave Jiang <> | Date | Thu, 24 Sep 2020 08:07:44 -0700 |
| |
On 9/24/2020 6:07 AM, Borislav Petkov wrote: > On Wed, Sep 23, 2020 at 04:10:43PM -0700, Dave Jiang wrote: >> +/* The dst parameter must be 64-bytes aligned */ >> +static inline void movdir64b(void *dst, const void *src) >> +{ >> + /* >> + * Note that this isn't an "on-stack copy", just definition of "dst" >> + * as a pointer to 64-bytes of stuff that is going to be overwritten. >> + * In the MOVDIR64B case that may be needed as you can use the >> + * MOVDIR64B instruction to copy arbitrary memory around. This trick >> + * lets the compiler know how much gets clobbered. >> + */ >> + volatile struct { char _[64]; } *__dst = dst; >> + >> + /* MOVDIR64B [rdx], rax */ >> + asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02" >> + : >> + : "m" (*(struct { char _[64];} **)src), "a" (__dst) >> + : "memory"); >> +} > > Ok, Micha and I hashed it out on IRC, here's what you do. Please keep > the comments too because we will forget soon again. > > static inline void movdir64b(void *__dst, const void *src) > { > struct { char _[64]; } *__src = src; > struct { char _[64]; } *__dst = dst; > > /* > * MOVDIR64B %(rdx), rax. > * > * Both __src and __dst must be memory constraints in order to tell the > * compiler that no other memory accesses should be reordered around > * this one. > * > * Also, both must be supplied as lvalues because this tells > * the compiler what the object is (its size) the instruction accesses. > * I.e., not the pointers but what they point, thus the deref'ing '*'. > */ > asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02" > : "+m" (*__dst) > : "m" (*__src), "a" (__dst), "d" (__src)); > }
Thanks Boris. I will update and resend.
> > Thx. >
| |