From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: XDP performance regression due to CONFIG_RETPOLINE Spectre V2 Date: Sat, 14 Apr 2018 21:29:26 +0200 Message-ID: <1523734166.15648.5.camel@infradead.org> References: <20180412155029.0324fe58@redhat.com> <20180412145123.GA7048@lst.de> <20180412145653.GA7172@lst.de> <20180413172611.GA23634@lst.de> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-CwrDHV4RQ2GXYoQ4SWaf" Cc: Jesper Dangaard Brouer , "xdp-newbies@vger.kernel.org" , "netdev@vger.kernel.org" , William Tu , =?ISO-8859-1?Q?Bj=F6rn_T=F6pel?= , "Karlsson, Magnus" , Alexander Duyck , Arnaldo Carvalho de Melo To: Christoph Hellwig , Tushar Dave Return-path: Received: from twosheds.infradead.org ([90.155.92.209]:49966 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751590AbeDNT37 (ORCPT ); Sat, 14 Apr 2018 15:29:59 -0400 In-Reply-To: <20180413172611.GA23634@lst.de> Sender: netdev-owner@vger.kernel.org List-ID: --=-CwrDHV4RQ2GXYoQ4SWaf Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-04-13 at 19:26 +0200, Christoph Hellwig wrote: > On Fri, Apr 13, 2018 at 10:12:41AM -0700, Tushar Dave wrote: > > I guess there is nothing we need to do! > > > > On x86, in case of no intel iommu or iommu is disabled, you end up in > > swiotlb for DMA API calls when system has 4G memory. > > However, AFAICT, for 64bit DMA capable devices swiotlb DMA APIs do not > > use bounce buffer until and unless you have swiotlb=3Dforce specified i= n > > kernel commandline. >=20 > Sure.=C2=A0 But that means very sync_*_to_device and sync_*_to_cpu now > involves an indirect call to do exactly nothing, which in the workload > Jesper is looking at is causing a huge performance degradation due to > retpolines. We should look at using the =C2=A0if (dma_ops =3D=3D swiotlb_dma_ops) =C2=A0 =C2=A0 swiotlb_map_page() =C2=A0else =C2=A0 =C2=A0 dma_ops->map_page() trick for this. Perhaps with alternatives so that when an Intel or AMD IOMMU is detected, it's *that* which is checked for as the special case. --=-CwrDHV4RQ2GXYoQ4SWaf Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQDgzLNWa18DNpGUj/ZeH0Sgz53ESIbzdPw3OJeuNP6jZhxZojbyfxbM hETscxI/Hj6UZ4a7sHm5BkVjlsB1Af2Za/PXUt8MmLAcPMHkMPGunvkUibEvblDvpqMkQZlaZM+t 5PqFmWkbehLaEvbpNY7dmEAAeKh4klTzJzrr5AAzaCQ32cA2e3+DEIv5O5l9ViMIjy/JM+xMQrfX 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