On 22.08.2025 00:56, Andrew Cooper wrote:
> On 15/08/2025 10:10 am, Jan Beulich wrote:
>> On 14.08.2025 22:55, Andrew Cooper wrote:
>>> On 14/08/2025 4:35 pm, Jan Beulich wrote:
>>>> On 08.08.2025 22:23, Andrew Cooper wrote:
>>>>> @@ -154,7 +155,6 @@ unsigned long get_stack_dump_bottom (unsigned long
>>>>> sp);
>>>>> "rdsspd %[ssp];" \
>>>>> "cmp $1, %[ssp];" \
>>>>> "je .L_shstk_done.%=;" /* CET not active? Skip. */ \
>>>>> - "mov $%c[skstk_base], %[val];" \
>>>>> "and $%c[stack_mask], %[ssp];" \
>>>>> "sub %[ssp], %[val];" \
>>>>> "shr $3, %[val];" \
>>>> With the latter two insns here, ...
>>>>
>>>>> @@ -177,6 +177,8 @@ unsigned long get_stack_dump_bottom (unsigned long
>>>>> sp);
>>>>>
>>>>> #define switch_stack_and_jump(fn, instr, constr) \
>>>>> ({ \
>>>>> + unsigned int token_offset = \
>>>>> + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - (opt_fred ? 0 : 8); \
>>>>> unsigned int tmp; \
>>>>> BUILD_BUG_ON(!ssaj_has_attr_noreturn(fn)); \
>>>>> __asm__ __volatile__ ( \
>>>>> @@ -184,12 +186,11 @@ unsigned long get_stack_dump_bottom (unsigned long
>>>>> sp);
>>>>> "mov %[stk], %%rsp;" \
>>>>> CHECK_FOR_LIVEPATCH_WORK \
>>>>> instr "[fun]" \
>>>>> - : [val] "=&r" (tmp), \
>>>>> + : [val] "=r" (tmp), \
>>>> ... I don't think you can legitimately drop the & from here? With it
>>>> retained:
>>>> Reviewed-by: Jan Beulich <jbeul...@suse.com>
>>> You chopped the bit which has an explicit input for "[val]", making the
>>> earlyclobber incorrect.
>> I was wondering whether there was a connection there, but ...
>>
>>> IIRC, one version of Clang complained.
>> ... that's not good. Without the early-clobber the asm() isn't quite
>> correct imo. If the same value appeared as another input, the compiler
>> may validly tie both together, assuming the register stays intact until
>> the very last insn (and hence even that last insn could still use the
>> register as an input). IOW if there's a Clang issue here, I think it
>> may need working around explicitly.
>
> Given that I need an alternative anyway, this becomes much easier, and
> shrinks to this single hunk:
>
> diff --git a/xen/arch/x86/include/asm/current.h
> b/xen/arch/x86/include/asm/current.h
> index c1eb27b1c4c2..35cc61fa88e7 100644
> --- a/xen/arch/x86/include/asm/current.h
> +++ b/xen/arch/x86/include/asm/current.h
> @@ -154,7 +154,9 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
> "rdsspd %[ssp];" \
> "cmp $1, %[ssp];" \
> "je .L_shstk_done.%=;" /* CET not active? Skip. */ \
> - "mov $%c[skstk_base], %[val];" \
> + ALTERNATIVE("mov $%c[skstk_base], %[val];", \
> + "mov $%c[skstk_base] + 8, %[val];", \
> + X86_FEATURE_XEN_FRED) \
> "and $%c[stack_mask], %[ssp];" \
> "sub %[ssp], %[val];" \
> "shr $3, %[val];" \
Oh, okay. But then please again without unnecessary use of $%c constructs,
when just % will do.
Tangential: Now that I look at this again, what's the 1st 'k' standing
for in skstk_base? Was that maybe meant to be 'h'?
Jan
Jan
