Re: [PATCH v6 01/15] x86: Add an x86_xsave_layout structure to handle variable XSAVE layouts.

John Baldwin
2023-07-26T21:27:59Z

On 7/26/23 12:22 PM, Simon Marchi wrote:
> On 7/14/23 11:51, John Baldwin wrote:
>> The standard layout of the XSAVE extended state area consists of three
>> regions. The first 512 bytes (legacy region) match the layout of the
>> FXSAVE instruction including floating point registers, MMX registers,
>> and SSE registers. The next 64 bytes (XSAVE header) contains a header
>> with a fixed layout. The final region (extended region) contains zero
>> or more optional state components. Examples of these include the
>> upper 128 bits of YMM registers for AVX.
>>
>> These optional state components generally have an
>> architecturally-fixed size, but they are not assigned architectural
>> offsets in the extended region. Instead, processors provide
>> additional CPUID leafs describing the size and offset of each
>> component in the "standard" layout for a given CPU. (There is also a
>> "compact" format which uses an alternate layout, but existing OS's
>> currently export the "standard" layout when exporting XSAVE data via
>> ptrace() and core dumps.)
>>
>> To date, GDB has assumed the layout used on current Intel processors
>> for state components in the extended region and hardcoded those
>> offsets in the tables in i387-tdep.c and i387-fp.cc. However, this
>> fails on recent AMD processors which use a different layout.
>> Specifically, AMD Zen3 and later processors do not leave space for the
>> MPX register set in between the AVX and AVX512 register sets.
>>
>> To rectify this, add an x86_xsave_layout structure which contains the
>> total size of the XSAVE extended state area as well as the offset of
>> each known optional state component.
>>
>> Subsequent commits will modify XSAVE parsing in both gdb and gdbserver
>> to use x86_xsave_layout.
>>
>> Co-authored-by: Aleksandar Paunovic <aleksandar.paunovic@intel.com>
>> ---
>> gdbsupport/x86-xstate.h | 65 +++++++++++++++++++++++++++++++++++------
>> 1 file changed, 56 insertions(+), 9 deletions(-)
>>
>> diff --git a/gdbsupport/x86-xstate.h b/gdbsupport/x86-xstate.h
>> index b8740fd8701..27fc0bd12f2 100644
>> --- a/gdbsupport/x86-xstate.h
>> +++ b/gdbsupport/x86-xstate.h
>> @@ -20,22 +20,69 @@
>> #ifndef COMMON_X86_XSTATE_H
>> #define COMMON_X86_XSTATE_H
>>
>> +/* The extended state feature IDs in the state component bitmap. */
>> +#define X86_XSTATE_X87_ID 0
>> +#define X86_XSTATE_SSE_ID 1
>> +#define X86_XSTATE_AVX_ID 2
>> +#define X86_XSTATE_BNDREGS_ID 3
>> +#define X86_XSTATE_BNDCFG_ID 4
>> +#define X86_XSTATE_K_ID 5
>> +#define X86_XSTATE_ZMM_H_ID 6
>> +#define X86_XSTATE_ZMM_ID 7
>> +#define X86_XSTATE_PKRU_ID 9
>> +
>> /* The extended state feature bits. */
>> -#define X86_XSTATE_X87 (1ULL << 0)
>> -#define X86_XSTATE_SSE (1ULL << 1)
>> -#define X86_XSTATE_AVX (1ULL << 2)
>> -#define X86_XSTATE_BNDREGS (1ULL << 3)
>> -#define X86_XSTATE_BNDCFG (1ULL << 4)
>> +#define X86_XSTATE_X87 (1ULL << X86_XSTATE_X87_ID)
>> +#define X86_XSTATE_SSE (1ULL << X86_XSTATE_SSE_ID)
>> +#define X86_XSTATE_AVX (1ULL << X86_XSTATE_AVX_ID)
>> +#define X86_XSTATE_BNDREGS (1ULL << X86_XSTATE_BNDREGS_ID)
>> +#define X86_XSTATE_BNDCFG (1ULL << X86_XSTATE_BNDCFG_ID)
>> #define X86_XSTATE_MPX (X86_XSTATE_BNDREGS | X86_XSTATE_BNDCFG)
>>
>> /* AVX 512 adds three feature bits. All three must be enabled. */
>> -#define X86_XSTATE_K (1ULL << 5)
>> -#define X86_XSTATE_ZMM_H (1ULL << 6)
>> -#define X86_XSTATE_ZMM (1ULL << 7)
>> +#define X86_XSTATE_K (1ULL << X86_XSTATE_K_ID)
>> +#define X86_XSTATE_ZMM_H (1ULL << X86_XSTATE_ZMM_H_ID)
>> +#define X86_XSTATE_ZMM (1ULL << X86_XSTATE_ZMM_ID)
>> #define X86_XSTATE_AVX512 (X86_XSTATE_K | X86_XSTATE_ZMM_H \
>> | X86_XSTATE_ZMM)
>>
>> -#define X86_XSTATE_PKRU (1ULL << 9)
>> +#define X86_XSTATE_PKRU (1ULL << X86_XSTATE_PKRU_ID)
>> +
>> +/* Size and offsets of register states in the XSAVE area extended
>> + region. Offsets are set to 0 to indicate the absence of the
>> + associated registers. */
>
> Extreme comment nitpick. In "Size and offsets", one is singular and the
> other is plural. Should it be "Sizes and offsets", or "Size and
> offset"?

Ah, there's a single size and multiple offsets. Is this version clearer:

/* Total size of the XSAVE area extended region and offsets of
register states within the region. Offsets are set to 0 to
indicate the absence of the associated registers. */

--
John Baldwin