From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 04EB2188CC8 for ; Tue, 9 Jul 2024 13:58:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720533533; cv=none; b=e7oh/oj8JyZHrzR+TU/qvC3OfF2Xnznd5h7U/W3+aY98NRE85wFPeRz2QWt0515UVjNK+x9vSEAMxLbFC57dfD5bDDVnnDsXMe+Q+UIEH9Cl5K/yqe/hki191L463IB2QH/cA9qsJpYOWNrkF1jSfi1DmiegbRBMm3Z9zKwKFZM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720533533; c=relaxed/simple; bh=4GwK0wBUo08PrDLPTcwJSHbI6ytKzz57oxvuxLcXKy0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oFDLJ+ZZm4PDVA8DnEDlGo34ixb7r6sdgWVIHK+N/C4G8IH7p5uavdH/RBJyb9cTlJOKvTxHfhHUjxtxe0jHErj6t8iAdVliS3ngnUzZ+vjEOSdxg1EV6XgP0K5M94d6Z3/tgUFQG5eht3RthceTCU2SlZ3fJCB2CyVgmaNEhD4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com; spf=pass smtp.mailfrom=zytor.com; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b=bJHewCJn; arc=none smtp.client-ip=198.137.202.136 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=zytor.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b="bJHewCJn" Received: from [192.168.7.205] ([71.202.166.45]) (authenticated bits=0) by mail.zytor.com (8.17.2/8.17.1) with ESMTPSA id 469Dw8Li3514289 (version=TLSv1.3 cipher=TLS_AES_128_GCM_SHA256 bits=128 verify=NO); Tue, 9 Jul 2024 06:58:10 -0700 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.zytor.com 469Dw8Li3514289 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zytor.com; s=2024061501; t=1720533492; bh=uY8yj978GlXI2G3OqNP1RrGw/fehtNWKPpdBQghfhs0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=bJHewCJnM1SWeSO+LiFzmaVfZsEJ8ctQavejQK40NDpGKsOzOAikcnTWyZCOVbyXp SHUUnw52f0Q1ZzBZPlV3TFSaDQqEyx5hPilhS3m12V+NogShJDyUyWN0GeFfdCPPdi N7A0+cXSU72VzqwZ0woeVcOqSKxL+XNvaCg3LpvwnSudoQiRDhSDBuulevMoev+Wp6 yfzjq96SxOC6HT/SbHVA8nDtWW4LIrBvu+W37X18146yStB2IidJ/HOjEZzZcYmnjw PUxsGTy7J85OoBnpZP9RWrCxoPrO4isja3J9HW0kkFCg3Rlp+aQlkpf66rGlc8bVG/ zT9CGMOlx71uQ== Message-ID: <246c77ec-1911-4f43-9ce6-7e087b3a4562@zytor.com> Date: Tue, 9 Jul 2024 06:58:07 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/4] x86/fred: Write to FRED MSRs with wrmsrns() To: Borislav Petkov , Andrew Cooper Cc: "H. Peter Anvin" , dave.hansen@intel.com, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org, peterz@infradead.org, nik.borisov@suse.com, houwenlong.hwl@antgroup.com, Juergen Gross References: <9063b0fe-e8f3-44ff-b323-b2b6c338690f@intel.com> <172002205406.3280081.14523962650685954182@Ubuntu-2204-jammy-amd64-base> <15f56e6a-6edd-43d0-8e83-bb6430096514@citrix.com> <20240703161705.GAZoV5gQIgtORQeHdQ@fat_crate.local> <20240705094418.GAZofAcvelmnRzbkoG@fat_crate.local> <20240705134517.GAZof47bcaL5i2b4ju@fat_crate.local> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; keydata= xsDNBGUPz1cBDACS/9yOJGojBFPxFt0OfTWuMl0uSgpwk37uRrFPTTLw4BaxhlFL0bjs6q+0 2OfG34R+a0ZCuj5c9vggUMoOLdDyA7yPVAJU0OX6lqpg6z/kyQg3t4jvajG6aCgwSDx5Kzg5 Rj3AXl8k2wb0jdqRB4RvaOPFiHNGgXCs5Pkux/qr0laeFIpzMKMootGa4kfURgPhRzUaM1vy bsMsL8vpJtGUmitrSqe5dVNBH00whLtPFM7IbzKURPUOkRRiusFAsw0a1ztCgoFczq6VfAVu raTye0L/VXwZd+aGi401V2tLsAHxxckRi9p3mc0jExPc60joK+aZPy6amwSCy5kAJ/AboYtY VmKIGKx1yx8POy6m+1lZ8C0q9b8eJ8kWPAR78PgT37FQWKYS1uAroG2wLdK7FiIEpPhCD+zH wlslo2ETbdKjrLIPNehQCOWrT32k8vFNEMLP5G/mmjfNj5sEf3IOKgMTMVl9AFjsINLHcxEQ 6T8nGbX/n3msP6A36FDfdSEAEQEAAc0WWGluIExpIDx4aW5Aenl0b3IuY29tPsLBDQQTAQgA NxYhBIUq/WFSDTiOvUIqv2u9DlcdrjdRBQJlD89XBQkFo5qAAhsDBAsJCAcFFQgJCgsFFgID AQAACgkQa70OVx2uN1HUpgv/cM2fsFCQodLArMTX5nt9yqAWgA5t1srri6EgS8W3F+3Kitge tYTBKu6j5BXuXaX3vyfCm+zajDJN77JHuYnpcKKr13VcZi1Swv6Jx1u0II8DOmoDYLb1Q2ZW v83W55fOWJ2g72x/UjVJBQ0sVjAngazU3ckc0TeNQlkcpSVGa/qBIHLfZraWtdrNAQT4A1fa sWGuJrChBFhtKbYXbUCu9AoYmmbQnsx2EWoJy3h7OjtfFapJbPZql+no5AJ3Mk9eE5oWyLH+ QWqtOeJM7kKvn/dBudokFSNhDUw06e7EoVPSJyUIMbYtUO7g2+Atu44G/EPP0yV0J4lRO6EA wYRXff7+I1jIWEHpj5EFVYO6SmBg7zF2illHEW31JAPtdDLDHYcZDfS41caEKOQIPsdzQkaQ oW2hchcjcMPAfyhhRzUpVHLPxLCetP8vrVhTvnaZUo0xaVYb3+wjP+D5j/3+hwblu2agPsaE vgVbZ8Fx3TUxUPCAdr/p73DGg57oHjgezsDNBGUPz1gBDAD4Mg7hMFRQqlzotcNSxatlAQNL MadLfUTFz8wUUa21LPLrHBkUwm8RujehJrzcVbPYwPXIO0uyL/F///CogMNx7Iwo6by43KOy g89wVFhyy237EY76j1lVfLzcMYmjBoTH95fJC/lVb5Whxil6KjSN/R/y3jfG1dPXfwAuZ/4N cMoOslWkfZKJeEut5aZTRepKKF54T5r49H9F7OFLyxrC/uI9UDttWqMxcWyCkHh0v1Di8176 jjYRNTrGEfYfGxSp+3jYL3PoNceIMkqM9haXjjGl0W1B4BidK1LVYBNov0rTEzyr0a1riUrp Qk+6z/LHxCM9lFFXnqH7KWeToTOPQebD2B/Ah5CZlft41i8L6LOF/LCuDBuYlu/fI2nuCc8d m4wwtkou1Y/kIwbEsE/6RQwRXUZhzO6llfoN96Fczr/RwvPIK5SVMixqWq4QGFAyK0m/1ap4 bhIRrdCLVQcgU4glo17vqfEaRcTW5SgX+pGs4KIPPBE5J/ABD6pBnUUAEQEAAcLA/AQYAQgA JhYhBIUq/WFSDTiOvUIqv2u9DlcdrjdRBQJlD89ZBQkFo5qAAhsMAAoJEGu9DlcdrjdR4C0L /RcjolEjoZW8VsyxWtXazQPnaRvzZ4vhmGOsCPr2BPtMlSwDzTlri8BBG1/3t/DNK4JLuwEj OAIE3fkkm+UG4Kjud6aNeraDI52DRVCSx6xff3bjmJsJJMb12mWglN6LjdF6K+PE+OTJUh2F dOhslN5C2kgl0dvUuevwMgQF3IljLmi/6APKYJHjkJpu1E6luZec/lRbetHuNFtbh3xgFIJx 2RpgVDP4xB3f8r0I+y6ua+p7fgOjDLyoFjubRGed0Be45JJQEn7A3CSb6Xu7NYobnxfkwAGZ Q81a2XtvNS7Aj6NWVoOQB5KbM4yosO5+Me1V1SkX2jlnn26JPEvbV3KRFcwV5RnDxm4OQTSk PYbAkjBbm+tuJ/Sm+5Yp5T/BnKz21FoCS8uvTiziHj2H7Cuekn6F8EYhegONm+RVg3vikOpn gao85i4HwQTK9/D1wgJIQkdwWXVMZ6q/OALaBp82vQ2U9sjTyFXgDjglgh00VRAHP7u1Rcu4 l75w1xInsg== In-Reply-To: <20240705134517.GAZof47bcaL5i2b4ju@fat_crate.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 7/5/2024 6:45 AM, Borislav Petkov wrote: > On Fri, Jul 05, 2024 at 11:30:16AM +0100, Andrew Cooper wrote: >> You cite perf.  Look at the disassembly of the two approaches... >> >> cpu_feature_enabled() might give you warm fuzzy feelings that you've >> eekd out every ounce of performance, but it's an absolute disaster at a >> code generation level by forcing the compiler to lay out both side and >> preventing any kind of CSE.  As I've reported before, count the number >> of RDPKRU instructions in trivial-looking xsave handling functions for a >> glimpse of the practical consequences. > > Yes, I do cite perf because what you have above is not saying: "yes, this is > a fast path and doing an alternative is warranted." If that is the case, sure, > by all means. If not, make the C readable and ignore code generation. Who > cares. > >> Anyway, none of this is the complicated aspect.  The complicated issue >> is the paravirt wrmsr(). >> >> TGLX's complaint is that everyone turns on CONFIG_PARAVIRT, and the >> paravirt hook for wmsr() is a code generation disaster WRT parameter >> handling.  I agree that it's not great, although it's got nothing on the >> damage done by cpu_feature_enabled(). >> >> >> But, seeing as I've got everyone's attention, I'll repeat my proposal >> for fixing this nicely, in the hope of any feedback on the 3rd posting... >> >> The underlying problem is that parameter setup for the paravirt wrmsr() >> follows a C calling convention, so the index/data are manifested into >> %rdi/%rsi.  Then, the out-line "native" hook shuffles the index/data >> back into %ecx/%edx/%eax, and this cost is borne in all kernels. > > A handful of reg ops per a WRMSRNS? Meh, same argument as above. But... > >> Instead, the better way would be to have a hook with a non-standard >> calling convention which happens to match the WRMSR instruction. >> >> That way, the native, and simple paravirt paths inline to a single >> instruction with no extraneous parameter shuffling, and the shuffling >> cost is borne by PARAVIRT_XXL only, where a reg/reg move is nothing >> compared to the hypercall involved. >> >> The only complication is the extable #GP hook, but that's fine to place >> at the paravirt site as long as the extable handler confirms the #GP >> came from a WRMSR{NS,} and not a branch. > > ... yes, I'd gladly review patches which address that and make the whole deal > cleaner. I'm still sceptical those handful of regs shuffling ops would matter > in any benchmark but sure, if it can be done in a cleaner way, why not... > > Unless I'm missing some use case where that overhead really matters. Then by > all means... > It looks that it no longer makes sense to include this patch in this patchset; it is not something that can be done as a small cleanup. Any objection? Thanks! Xin