From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH v2] x86/hvm: re-work viridian APIC assist code Date: Wed, 05 Sep 2018 11:45:00 +0100 Message-ID: <1536144300.31549.63.camel@infradead.org> References: <20180118151059.1336-1-paul.durrant@citrix.com> <1535153880.24926.28.camel@infradead.org> <1536093061.31549.47.camel@infradead.org> <88abe6b0dee947ad93af543653dde12a@AMSPEX02CL03.citrite.net> <1536140422.31549.59.camel@infradead.org> <2135f2c459ee45e2a038a9fbc9145392@AMSPEX02CL03.citrite.net> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============7200263426876998995==" Return-path: Received: from us1-rack-dfw2.inumbo.com ([104.130.134.6]) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1fxVJ4-000767-HK for xen-devel@lists.xenproject.org; Wed, 05 Sep 2018 10:45:06 +0000 In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" To: Paul Durrant , "xen-devel@lists.xenproject.org" Cc: Eslam Elnikety , Andrew Cooper , Shan Haitao , Jan Beulich List-Id: xen-devel@lists.xenproject.org --===============7200263426876998995== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-PC83/da9YzmSMuKVLl3b" --=-PC83/da9YzmSMuKVLl3b Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2018-09-05 at 10:40 +0000, Paul Durrant wrote: >=20 > Actually the neatest approach would be to get information into the > vlapic code as to whether APIC assist is suitable for the given > vector so that the code there can selectively enable it, and then Xen > would know it was safe to avoid fully emulating an EOI for anything > that did have assist enabled. I'm not sure I understand why an assisted EOI should be any different from "normal" EOI. The global lock and indirect function calls and all that stuff that hvm_dpci_msi_eoi() does are *expensive*, for a rare case. Why not bypass all that when it's not needed, for "normal" EOI and APIC-assisted EOI alike? 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