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Re: [PATCH 20/22] x86/traps: Alter switch_stack_and_jump() for FRED mode


  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Mon, 25 Aug 2025 11:19:02 +0200
  • Autocrypt: addr=jbeulich@xxxxxxxx; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL
  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 25 Aug 2025 09:19:09 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

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 <jbeulich@xxxxxxxx>
>>> 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



 


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