From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Luis R. Rodriguez" Subject: Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to paravirt_legacy Date: Tue, 9 Feb 2016 07:22:37 +0100 Message-ID: <20160209062237.GH25240@wotan.suse.de> References: <56B8B6BF.6030007@oracle.com> <20160208155507.GF28980@pd.tnic> <56B8BCDB.9040701@citrix.com> <56B8BE58.6070003@oracle.com> <56B8C1BC.9040603@citrix.com> <56B8C2C8.8050406@oracle.com> <20160208163526.GI28980@pd.tnic> <56B8C490.1060004@citrix.com> <20160208164545.GJ28980@pd.tnic> <56B8C7EC.3090403@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <56B8C7EC.3090403@citrix.com> Sender: linux-kernel-owner@vger.kernel.org To: Andrew Cooper Cc: Borislav Petkov , Boris Ostrovsky , X86 ML , david.e.box@intel.com, Andrey Ryabinin , Lv Zheng , Jan Beulich , "H. Peter Anvin" , qiuxishi@huawei.com, cocci@systeme.lip6.fr, Xen Devel , Joerg Roedel , "Rafael J. Wysocki" , Robert Moore , Ingo Molnar , Andrey Ryabinin , Mauro Carvalho Chehab , Rusty Russell , Thomas Gleixner , mcb30@ipxe.org, Juergen Gross , "linux-kernel@vger.kernel.org" , Andy Lutomirski , "Luis R. Rodriguez" , long.wanglo List-Id: xen-devel@lists.xenproject.org On Mon, Feb 08, 2016 at 04:53:00PM +0000, Andrew Cooper wrote: > On 08/02/16 16:45, Borislav Petkov wrote: > > On Mon, Feb 08, 2016 at 04:38:40PM +0000, Andrew Cooper wrote: > >> Does the early loader have extable support? If so, this is fairly easy > >> to fix. If not, we have a problem. > > It doesn't and regardless, you want to have this CPUID querying as > > simple as possible. No special handling, no special prefixes as it > > should be able to run on other hypervisors too. > > > > If one can't execute a simple CPUID(0x4...) on a xen guest and get the > > results back, then for early, we will have to do what we've done until > > now and simply emulate the MSR accesses. > > > > Later code can use then xen_cpuid() and all is fine. We should still get > > rid of paravirt_enabled() though. > > > > The force emulation prefix starts with a ud2a instruction, so extable is > to prevent it breaking on non-Xen systems. However, if extable isn't > available, this point is moot. > > As an alternative check which should be doable this early on, peeking in > the head of hypercall_page should work. If Linux was booted as a PV > guest, the hypercall_page will have been constructed by the domain > builder, and won't have 0x90's in it. Most of the paravirt_enabled() checks can be replaced with a hardware_subarch check once we modify the xen_start_kernel() to add that, today XEN is unused even though it was added eons ago. Part of my work was to remove as many paravirt_enabled() checks. I'll reply to Andy's original e-mail now indicating which ones I could address, it sounds like with this nugget and some other work we might be able to address all. I should note, in the future the check for subarch would be an explicit part of the x86 early init init routines to avoid further issues but only in between x86_64_start_reservations() and setup_arch(). How *early* can such a hypercall _page check be *safely* be called? I say safely here as if we're not on Xen are we OK to muck around and check the same address space? Provided we use the subarch for PV to remove a lot of the paravirt_enabled() checks, is HVMLite still OK if it ends up using PC subarch and there not being a paravirt_enabled() anyamore? Boris O's HVMLite series added paravirt_enabled = 1 for the new HVMLite. Luis