Re: [PATCH v4 04/13] x86 nat: Add helper functions to save the XSAVE layout for the host.

[John Baldwin <jhb@FreeBSD.org>]

On 4/6/23 12:40 PM, Simon Marchi wrote:
> On 3/17/23 21:08, John Baldwin wrote:
>> x86_xsave_length returns the total length of the XSAVE state area
>> standard format as queried from CPUID.
>>
>> x86_fetch_xsave_layout uses CPUID to query the offsets of XSAVE
>> extended regions from the running host.  The total length of the XSAVE
>> state area can either be supplied the caller if known (e.g. from
>> FreeBSD's PT_GETXSTATEINFO) or it can be queried from the running host
>> using x86_xsave_length.
>> ---
>>   gdb/nat/x86-xstate.c | 65 ++++++++++++++++++++++++++++++++++++++++++++
>>   gdb/nat/x86-xstate.h | 35 ++++++++++++++++++++++++
>>   2 files changed, 100 insertions(+)
>>   create mode 100644 gdb/nat/x86-xstate.c
>>   create mode 100644 gdb/nat/x86-xstate.h
>>
>> diff --git a/gdb/nat/x86-xstate.c b/gdb/nat/x86-xstate.c
>> new file mode 100644
>> index 00000000000..7b42efe1266
>> --- /dev/null
>> +++ b/gdb/nat/x86-xstate.c
>> @@ -0,0 +1,65 @@
>> +/* x86 XSAVE extended state functions.
>> +
>> +   Copyright (C) 2022-2023 Free Software Foundation, Inc.
>> +
>> +   This file is part of GDB.
>> +
>> +   This program is free software; you can redistribute it and/or modify
>> +   it under the terms of the GNU General Public License as published by
>> +   the Free Software Foundation; either version 3 of the License, or
>> +   (at your option) any later version.
>> +
>> +   This program is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +   GNU General Public License for more details.
>> +
>> +   You should have received a copy of the GNU General Public License
>> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
>> +
>> +#include "gdbsupport/common-defs.h"
>> +#include "gdbsupport/x86-xstate.h"
>> +#include "nat/x86-cpuid.h"
>> +#include "nat/x86-xstate.h"
>> +
>> +/* Fetch the offset of a specific XSAVE extended region.  */
>> +
>> +static int
>> +xsave_feature_offset (uint64_t xcr0, int feature)
>> +{
>> +  uint32_t ebx;
>> +
>> +  if ((xcr0 & (1ULL << feature)) == 0)
>> +    return 0;
>> +
>> +  if (!x86_cpuid_count (0xd, feature, nullptr, &ebx, nullptr, nullptr))
> 
> Does x86_cpuid_count return a count, as the name suggests (and not a
> boolean)?  If so, it should be == 0.  Otherwise, ignore that.

No, it returns a bool like x86_cpuid.  Some CPUID leaves on x86 take a
sub-leaf where the main leaf is specified in EAX and the sub-leaf in ECX
when CPUID is invoked.  The first CPUID leaves that used this documented
the ECX input as a count (I think the first ones were used for enumerating
cache topology info where ECX was basically an index).  As a result, the
APIs for CPUID builtins in GCC, etc. all use the suffix "_count" for the
variants that take the ECX sub-leaf as an input.

>> +/* See x86-xstate.h.  */
>> +
>> +void
>> +x86_fetch_xsave_layout (uint64_t xcr0, int len, x86_xsave_layout &layout)
>> +{
>> +  layout.sizeof_xsave = len;
>> +  layout.avx_offset = xsave_feature_offset(xcr0, X86_XSTATE_AVX_ID);
>> +  layout.bndregs_offset = xsave_feature_offset(xcr0, X86_XSTATE_BNDREGS_ID);
>> +  layout.bndcfg_offset = xsave_feature_offset(xcr0, X86_XSTATE_BNDCFG_ID);
>> +  layout.k_offset = xsave_feature_offset(xcr0, X86_XSTATE_K_ID);
>> +  layout.zmm_h_offset = xsave_feature_offset(xcr0, X86_XSTATE_ZMM_H_ID);
>> +  layout.zmm_offset = xsave_feature_offset(xcr0, X86_XSTATE_ZMM_ID);
>> +  layout.pkru_offset = xsave_feature_offset(xcr0, X86_XSTATE_PKRU_ID);
> 
> Missing spaces.

Oops, fixed.

>> diff --git a/gdb/nat/x86-xstate.h b/gdb/nat/x86-xstate.h
>> new file mode 100644
>> index 00000000000..e9f5fb57e3a
>> --- /dev/null
>> +++ b/gdb/nat/x86-xstate.h
>> @@ -0,0 +1,35 @@
>> +/* x86 XSAVE extended state functions.
>> +
>> +   Copyright (C) 2022-2023 Free Software Foundation, Inc.
>> +
>> +   This file is part of GDB.
>> +
>> +   This program is free software; you can redistribute it and/or modify
>> +   it under the terms of the GNU General Public License as published by
>> +   the Free Software Foundation; either version 3 of the License, or
>> +   (at your option) any later version.
>> +
>> +   This program is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +   GNU General Public License for more details.
>> +
>> +   You should have received a copy of the GNU General Public License
>> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
>> +
>> +#ifndef NAT_X86_XSTATE_H
>> +#define NAT_X86_XSTATE_H
>> +
>> +#include "gdbsupport/x86-xstate.h"
>> +
>> +/* Return the size of the XSAVE extended state fetched via CPUID.  */
>> +
>> +int x86_xsave_length ();
>> +
>> +/* Save the size and offsets of the XSAVE extended regions for the
>> +   running host in LAYOUT.  Offsets of each of the enabled regions in
>> +   XCR0 are fetched via CPUID.  */
>> +
>> +void x86_fetch_xsave_layout (uint64_t xcr0, int len, x86_xsave_layout &layout);
> 
> The function could maybe return an x86_xsave_layout.

Ah, fair enough.

-- 
John Baldwin