From owner-freebsd-arm@freebsd.org Thu Sep 10 15:06:34 2015 Return-Path: Delivered-To: freebsd-arm@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 271F5A01C22 for ; Thu, 10 Sep 2015 15:06:34 +0000 (UTC) (envelope-from JBeulich@suse.com) Received: from prv-mh.provo.novell.com (prv-mh.provo.novell.com [137.65.248.74]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "*.provo.novell.com", Issuer "DigiCert SHA2 High Assurance Server CA" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id EA2B61CFB for ; Thu, 10 Sep 2015 15:06:33 +0000 (UTC) (envelope-from JBeulich@suse.com) Received: from INET-PRV-MTA by prv-mh.provo.novell.com with Novell_GroupWise; Thu, 10 Sep 2015 09:06:32 -0600 Message-Id: <55F1B89802000078000A1C9B@prv-mh.provo.novell.com> X-Mailer: Novell GroupWise Internet Agent 14.0.1 Date: Thu, 10 Sep 2015 09:06:32 -0600 From: "Jan Beulich" To: "Mark Rutland" Cc: "Ian.Campbell@citrix.com" , "julien.grall@citrix.com" , "Stefano Stabellini" , "freebsd-arm@freebsd.org" , "peter.huangpeng@huawei.com" , "Shannon Zhao" , "matt.fleming@intel.com" , "ard.biesheuvel@linaro.org" , "christoffer.dall@linaro.org" , "leif.lindholm@linaro.org" , "shannon.zhao@linaro.org" , "linux-arm-kernel@lists.infradead.org" , "xen-devel@lists.xen.org" , "daniel.kiper@oracle.com" , "Konrad Rzeszutek Wilk" , "devicetree@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-efi@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] efi/libstub/fdt: Standardize the names of EFI stub parameters References: <1441874516-11364-1-git-send-email-zhaoshenglong@huawei.com> <20150910095208.GA29293@leverpostej> <20150910112418.GC29293@leverpostej> <55F199DD02000078000A1B1E@prv-mh.provo.novell.com> <20150910145331.GJ29293@leverpostej> In-Reply-To: <20150910145331.GJ29293@leverpostej> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-Mailman-Approved-At: Thu, 10 Sep 2015 17:20:49 +0000 X-BeenThere: freebsd-arm@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: "Porting FreeBSD to ARM processors." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Sep 2015 15:06:34 -0000 >>> On 10.09.15 at 16:53, wrote: > On Thu, Sep 10, 2015 at 01:55:25PM +0100, Jan Beulich wrote: >> >>> On 10.09.15 at 13:37, wrote: >> > On Thu, 10 Sep 2015, Mark Rutland wrote: >> >> Why can't Xen give a virtual EFI interface to Dom0 / guests? e.g. >> >> create pages of RuntimeServicesCode that are trivial assembly shims >> >> doing hypercalls, and plumb these into the virtual EFI memory map = and >> >> tables? >> >>=20 >> >> That would keep things sane for any guest, allow for easy addition = of >> >> EFI features, and you could even enter the usual EFI entry point, >> >> simulate ExitBootServices(), SetVirtualAddressMap(), and allow the = guest >> >> to make things sane for itself... >> >=20 >> > That's the way it was done on x86 and now we have common code both in >> > Linux (drivers/xen/efi.c) and Xen (xen/common/efi) which implement = this >> > scheme. Switching to a different solution for ARM, would mean = diverging >> > with x86, which is not nice, or reimplementing the x86 solution too, >> > which is expensive. >> >=20 >> > BTW I think that the idea you proposed was actually considered at the >> > time and deemed hard to implement, if I recall correctly. >>=20 >> Considering that the EFI support is just for Dom0, and Dom0 (at >> the time) had to be PV anyway, it was the more natural solution to >> expose the interface via hypercalls, the more that this allows better >> control over what is and primarily what is not being exposed to >> Dom0. With the wrapper approach we'd be back to the same >> problem (discussed elsewhere) of which EFI version to surface: The >> host one would impose potentially missing extensions, while the >> most recent hypervisor known one might imply hiding valuable >> information from Dom0. Plus there are incompatible changes like >> the altered meaning of EFI_MEMORY_WP in 2.5. >=20 > I'm not sure I follow how hypercalls solve any impedance mismatch here; > you're still expecting Dom0 to call up to Xen in order to perform calls, > and all I suggested was a different location for those hypercalls. >=20 > If Xen is happy to make such calls blindly, why does it matter if the > hypercall was in the kernel binary or an external shim? Because there could be new entries in SystemTable->RuntimeServices (expected and blindly but validly called by the kernel). Even worse (because likely harder to deal with) would be new fields in other structures. > Incompatible changes are a spec problem regardless of how this is > handled. Not necessarily - we don't expose the memory map (we'd have to if we were to mimic EFI for Dom0), and hence the mentioned issue doesn't exist in our model. Jan