asm() constraints need to fit both the intended insn(s) which the
respective operands are going to be used with as well as the actual kind
of value specified. "m" (alone) together with a constant, however, leads
to gcc saying
error: memory input <N> is not directly addressable
while clang complains
error: invalid lvalue in asm input for constraint 'm'
And rightly so - in order to access a memory operand, an address needs
to be specified to the insn. In some cases it might be possible for a
compiler to synthesize a memory operand holding the requested constant,
but I think any solution there would have sharp edges.
If "m" alone doesn't work with constants, it is at best pointless (and
perhaps misleading or even risky - the compiler might decide to actually
pick "m" and not try very hard to find a suitable register) to specify
it alongside "r".
Signed-off-by: Jan Beulich <jbeul...@suse.com>
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -736,7 +736,7 @@ void __init detect_zen2_null_seg_behavio
uint64_t base;
wrmsrl(MSR_FS_BASE, 1);
- asm volatile ( "mov %0, %%fs" :: "rm" (0) );
+ asm volatile ( "mov %0, %%fs" :: "r" (0) );
rdmsrl(MSR_FS_BASE, base);
if (base == 0)
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -248,7 +248,7 @@ void do_double_fault(struct cpu_user_reg
console_force_unlock();
- asm ( "lsll %1, %0" : "=r" (cpu) : "rm" (PER_CPU_SELECTOR) );
+ asm ( "lsll %1, %0" : "=r" (cpu) : "r" (PER_CPU_SELECTOR) );
/* Find information saved during fault and dump it to the console. */
printk("*** DOUBLE FAULT ***\n");
