Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <19b6f502-94f2-621a-4c30-fe9641474669@redhat.com>
Date:   Thu, 14 Jul 2022 19:12:00 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.10.0
Content-Language: en-US
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Boris Petkov <bp@alien8.de>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Florian Fainelli <f.fainelli@gmail.com>,
        kvm list <kvm@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        stable <stable@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Pavel Machek <pavel@denx.de>,
        Jon Hunter <jonathanh@nvidia.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Slade Watkins <slade@sladewatkins.com>, patches@kernelci.org,
        Sean Christopherson <seanjc@google.com>,
        Shuah Khan <shuah@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
        lkft-triage@lists.linaro.org,
        =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Anders Roxell <anders.roxell@linaro.org>
References: <20220712183238.844813653@linuxfoundation.org>
 <CA+G9fYtntg7=zWSs-dm+n_AUr_u0eBOU0zrwWqMeXZ+SF6_bLw@mail.gmail.com>
 <eb63e4ce-843f-c840-060e-6e15defd3c4d@roeck-us.net>
 <CAHk-=wj5cOA+fbGeV15kvwe6YGT54Wsk8F2UGoekVQLTPJz_pw@mail.gmail.com>
 <CAHk-=wgq1soM4gudypWLVQdYuvJbXn38LtvJMtnLZX+RTypqLg@mail.gmail.com>
 <Ys/bYJ2bLVfNBjFI@nazgul.tnic>
 <6b4337f4-d1de-7ba3-14e8-3ad0f9b18788@redhat.com>
 <8BEC3365-FC09-46C5-8211-518657C0308E@alien8.de>
 <CAHk-=wj4vtoWZPMXJU-B9qW1zLHsoA1Qb2P0NW=UFhZmrCrf9Q@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 5.15 00/78] 5.15.55-rc1 review
In-Reply-To: <CAHk-=wj4vtoWZPMXJU-B9qW1zLHsoA1Qb2P0NW=UFhZmrCrf9Q@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 7/14/22 19:02, Linus Torvalds wrote:
> And guess what? The code could just use roundup_pow_of_two(), which is
> designed exactly like ilog2() to be used for compile-time constant
> values.
> 
> So the code should just use
> 
>      #define FASTOP_SIZE roundup_pow_of_two(FASTOP_LENGTH)
> 
> and be a lot more legible, wouldn't it?
> 
> Because I don't think there is anything magical about the length
> "8/16/32". It's purely "aligned and big enough to contain
> FASTOP_LENGTH".

roundup_pow_of_two unfortunately is not enough for stringizing 
FASTOP_SIZE into an asm statement. :(

	#define __FOP_FUNC(name) \
	        ".align " __stringify(FASTOP_SIZE) " \n\t" \
	        ".type " name ", @function \n\t" \
	        name ":\n\t" \
	        ASM_ENDBR

The shifts are what we came up with for the SETCC thunks when ENDBR and 
SLS made them grew beyond 4 bytes; Peter's patch is reusing the trick 
for the fastop thunks.

> Because I don't think there is anything magical about the length
> "8/16/32". It's purely "aligned and big enough to contain
> FASTOP_LENGTH".

I agree with that, it's only limited to 8/16/32 to keep the macro to a 
decent size.

> And then the point of that
> 
>     static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
> 
> just goes away, because there are no subtle math issues there any more.
> 
> In fact, the remaining question is just "where did the 7 come from" in
> 
>     #define FASTOP_LENGTH (7 + ENDBR_INSN_SIZE + RET_LENGTH)

The 7 is an upper limit to the length of the code between endbr and ret.
There's no particular reason to limit to 7, but it allows using an 
alignment of 8 in the smallest case (no thunks, no SLS, no endbr) where 
you just have ".align 8; ...; ret".

Paolo