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: Tue, 04 Sep 2018 21:31:01 +0100 Message-ID: <1536093061.31549.47.camel@infradead.org> References: <20180118151059.1336-1-paul.durrant@citrix.com> <1535153880.24926.28.camel@infradead.org> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1666436183058756860==" 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 1fxHyg-00037T-QT for xen-devel@lists.xenproject.org; Tue, 04 Sep 2018 20:31:10 +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 --===============1666436183058756860== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-J8rCIxq36u6rukF/B5vL" --=-J8rCIxq36u6rukF/B5vL Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-09-03 at 10:12 +0000, Paul Durrant wrote: >=20 > I believe APIC assist is intended for fully synthetic interrupts. Hm, if by 'fully synthetic interrupts' you mean vlapic_virtual_intr_delivery_enabled(), then no I think APIC assist doesn't get used in that case at all. > Is it definitely this patch that causes the problem? It was only > intended to fix previous incorrectness but, if this is the culprit, > then it's clearly caused collateral damage in a logically unrelated > area. Not entirely. The performance gain we observed with APIC assist in the first place was basically stolen. It wasn't just bypassing the vmexit for that EOI; it was *so* much faster because it actually didn't ever do the EOI properly at all. You fixed that omission and unsurprisingly it got slower again; most of the apparent benefit of APIC assist is lost. But that's because it was never really doing the right thing in the first place. That EOI handling for unmaskable MSI is really painfully slow, so my=C2=A0 hack bypasses it in the common case where it isn't really necessary. FWIW I've done it in my tree with a single per-domain flag rather than a per-vector bitmap now, which makes it slightly simpler. --=-J8rCIxq36u6rukF/B5vL 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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