From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v2] x86: synchronize PCI config space access decoding Date: Wed, 17 Jun 2015 10:36:16 +0100 Message-ID: <55813F90.5020904@citrix.com> References: <557EFDC30200007800084F27@mail.emea.novell.com> <557EEFFF.7080807@citrix.com> <557FF5BE020000780008547C@mail.emea.novell.com> <55806A59.3080201@citrix.com> <55812FCD0200007800085D4C@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta5.messagelabs.com ([195.245.231.135]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1Z59ld-0003ea-Ky for xen-devel@lists.xenproject.org; Wed, 17 Jun 2015 09:36:21 +0000 In-Reply-To: <55812FCD0200007800085D4C@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: xen-devel , Keir Fraser List-Id: xen-devel@lists.xenproject.org On 17/06/15 07:29, Jan Beulich wrote: >>>> On 16.06.15 at 20:26, wrote: >> On 16/06/15 09:09, Jan Beulich wrote: >>>>>> On 15.06.15 at 17:32, wrote: >>>> On 15/06/15 15:30, Jan Beulich wrote: >>>>> @@ -2439,9 +2434,19 @@ struct hvm_ioreq_server *hvm_select_iore >>>>> >>>>> type = IOREQ_TYPE_PCI_CONFIG; >>>>> addr = ((uint64_t)sbdf << 32) | >>>>> - CF8_ADDR_HI(cf8) | >>>>> CF8_ADDR_LO(cf8) | >>>>> (p->addr & 3); >>>>> + /* AMD extended configuration space access? */ >>>>> + if ( CF8_ADDR_HI(cf8) && >>>>> + d->arch.x86_vendor == X86_VENDOR_AMD && >>>>> + d->arch.x86 >= 0x10 && d->arch.x86 <= 0x17 ) >>>>> + { >>>>> + uint64_t msr_val; >>>>> + >>>>> + if ( !rdmsr_safe(MSR_AMD64_NB_CFG, msr_val) && >>>> We now have several common paths which read this MSR looking for CF8_EXT. >>>> >>>> I think it would make sense to probe this on boot and have a >>>> cpu_has_amd_cf8_ext rather than repeatedly sampling an off-cpu MSR, >>>> although this would require synchronising it across all northbridges in >>>> emulate privileged op. >>>> >>>> Alternatively, it might just be better to unconditionally enable it >>>> during startup (as Linux does) and prevent dom0 from playing, which >>>> would avoid the need to synchronise updates to it. >>> You just repeat what you said for v1, without taking into >>> consideration my reply thereto: Us not using this method >>> ourselves, we should honor and play by what Dom0 does. >> Sorry - I had completely forgotten that this was a v2, and had already >> asked this question. >> >> However, hvm_select_ioreq_server() it not a rare function to call, and I >> am still concerned with the overhead. > And I can only repeat that the MSR isn't being accessed unless an > apparent extended access is being seen. Ok - I suppose that isn't too bad. Combined with below, it should never actually happen. > >> It turns out that MSR_AMD64_NB_CFG is unconditionally RAZ and has all >> writes discarded, so no HVM guest will ever be in a position to >> legitimately use AMD extended configuration access. > Where have you found that? The register (named NB_CFG1 in > newer families' BKGDs) is clearly r/w. It is implemented as RAZ/write discard in the hvm msr intercept code, and appears to exist only to prevent the guest blowing up in a cross-vendor case. ~Andrew