Re: [PATCH v6 5/5] x86/gsseg: use the LKGS instruction if available for load_gs_index()

["H. Peter Anvin" <hpa@zytor.com>]

On June 30, 2024 9:04:48 AM PDT, Borislav Petkov <bp@alien8.de> wrote:
>On Wed, Jan 11, 2023 at 11:20:32PM -0800, Xin Li wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>> 
>> The LKGS instruction atomically loads a segment descriptor into the
>> %gs descriptor registers, *except* that %gs.base is unchanged, and the
>> base is instead loaded into MSR_IA32_KERNEL_GS_BASE, which is exactly
>> what we want this function to do.
>> 
>> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Signed-off-by: Xin Li <xin3.li@intel.com>
>> ---
>> 
>> Changes since v5:
>> * Remove reviewers' SOBs (Borislav Petkov).
>> 
>> Changes since v4:
>> * Clear the LKGS feature from Xen PV guests (Juergen Gross).
>> 
>> Changes since v3:
>> * We want less ASM not more, thus keep local_irq_{save,restore}() inside
>>   native_load_gs_index() (Thomas Gleixner).
>> * For paravirt enabled kernels, initialize pv_ops.cpu.load_gs_index to
>>   native_lkgs (Thomas Gleixner).
>> 
>> Changes since v2:
>> * Mark DI as input and output (+D) as in v1, since the exception handler
>>   modifies it (Brian Gerst).
>> 
>> Changes since v1:
>> * Use EX_TYPE_ZERO_REG instead of fixup code in the obsolete .fixup code
>>   section (Peter Zijlstra).
>> * Add a comment that states the LKGS_DI macro will be replaced with "lkgs %di"
>>   once the binutils support the LKGS instruction (Peter Zijlstra).
>> ---
>>  arch/x86/include/asm/gsseg.h | 33 +++++++++++++++++++++++++++++----
>>  arch/x86/kernel/cpu/common.c |  1 +
>>  arch/x86/xen/enlighten_pv.c  |  1 +
>>  3 files changed, 31 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/gsseg.h b/arch/x86/include/asm/gsseg.h
>> index d15577c39e8d..ab6a595cea70 100644
>> --- a/arch/x86/include/asm/gsseg.h
>> +++ b/arch/x86/include/asm/gsseg.h
>> @@ -14,17 +14,42 @@
>>  
>>  extern asmlinkage void asm_load_gs_index(u16 selector);
>>  
>> +/* Replace with "lkgs %di" once binutils support LKGS instruction */
>> +#define LKGS_DI _ASM_BYTES(0xf2,0x0f,0x00,0xf7)
>
>Right, so this is %di only and not %edi.
>
>FRED spec says:
>
>"LKGS takes a single 16-bit operand (in a register or memory) and uses it to
>load a descriptor from the GDT or an LDT."
>
>However, latest objdump (GNU objdump (GNU Binutils) 2.42.50.20240630) says
>
>ffffffff81026dfb:       f2 0f 00 f7             lkgs   %edi
>
>objdump needs fixing, I'd say...
>

It is much like "movl %edi,%ds"...