User-Agent: Cyrus-JMAP/3.5.0-alpha0-4458-g51a91c06b2-fm-20211130.004-g51a91c06
Mime-Version: 1.0
Message-Id: <57665273-e56e-4b66-9dbd-003d592eb65d@www.fastmail.com>
In-Reply-To: <343c2c64-4d0b-6e21-80e0-834d0b7147aa@intel.com>
References: <20211203003636.11417-1-jiaxun.yang@flygoat.com>
 <20211203003636.11417-8-jiaxun.yang@flygoat.com>
 <343c2c64-4d0b-6e21-80e0-834d0b7147aa@intel.com>
Date:   Fri, 03 Dec 2021 11:39:19 +0000
From:   "Jiaxun Yang" <jiaxun.yang@flygoat.com>
To:     "Dave Hansen" <dave.hansen@intel.com>, x86@kernel.org
Cc:     "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        chang.seok.bae@intel.com, linux-kernel@vger.kernel.org,
        "Jiaxun Yang" <j.yang-87@sms.ed.ac.uk>
Subject: Re: [RFC PATCH 07/10] x86/fpu: Rellocate fpstate on save_fpregs_to_fpstate
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk


=E5=9C=A82021=E5=B9=B412=E6=9C=883=E6=97=A5=E5=8D=81=E4=BA=8C=E6=9C=88 =E4=
=B8=8A=E5=8D=8812:44=EF=BC=8CDave Hansen=E5=86=99=E9=81=93=EF=BC=9A
> On 12/2/21 4:36 PM, Jiaxun Yang wrote:
>> --- a/arch/x86/kernel/fpu/core.c
>> +++ b/arch/x86/kernel/fpu/core.c
>> @@ -112,6 +112,22 @@ static void update_avx_timestamp(struct fpu *fpu)
>>  		fpu->avx512_timestamp =3D jiffies;
>>  }
>> =20
>> +/* Update xstate size if it more dynamic features are opted-in. */
>> +static inline void xstate_update_size(struct fpu *fpu)
>> +{
>> +	int err;
>> +	struct fpstate *fpstate =3D fpu->fpstate;
>> +	u64 fpsmask =3D fpstate->xfeatures;
>> +	u64 curmask =3D fpsmask | xfeatures_in_use();
>> +
>> +	if (fpu_state_size_dynamic()) {
>> +		if (fpsmask !=3D curmask) {
>> +			err =3D fpstate_realloc(fpu, curmask);
>> +			WARN_ON_FPU(err);
>> +		}
>> +	}
>> +}
>> +
>>  /*
>>   * Save the FPU register state in fpu->fpstate->regs. The register s=
tate is
>>   * preserved.
>> @@ -129,6 +145,7 @@ static void update_avx_timestamp(struct fpu *fpu)
>>  void save_fpregs_to_fpstate(struct fpu *fpu)
>>  {
>>  	if (likely(use_xsave())) {
>> +		xstate_update_size(fpu);
>>  		os_xsave(fpu->fpstate);
>>  		update_avx_timestamp(fpu);
>>  		return;
>
> Have you considered what exactly happens when you hit that WARN_ON_FPU=
()
> which otherwise ignores the allocation error?  Have you considered what
> happens on the os_xsave() that follows it immediately?  How about what
> happens the next time this task runs after that failure?

Thank you for the catch.
This is a few questions that I don't have answer, so it's a RFC.

I thought it is unlikely to happen as kmalloc has emergency pool.
But in case it happens, I guess the best way to handle it is just
send SIGILL to corresponding user process or panic if it's kernel
fpu use?

Thanks.
--=20
- Jiaxun