Date: Thu, 10 Sep 2015 09:06:32 -0600 From: "Jan Beulich" <JBeulich@suse.com> To: "Mark Rutland" <mark.rutland@arm.com> Cc: "Ian.Campbell@citrix.com" <Ian.Campbell@citrix.com>, "julien.grall@citrix.com" <julien.grall@citrix.com>, "Stefano Stabellini" <stefano.stabellini@eu.citrix.com>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org>, "peter.huangpeng@huawei.com" <peter.huangpeng@huawei.com>, "Shannon Zhao" <zhaoshenglong@huawei.com>, "matt.fleming@intel.com" <matt.fleming@intel.com>, "ard.biesheuvel@linaro.org" <ard.biesheuvel@linaro.org>, "christoffer.dall@linaro.org" <christoffer.dall@linaro.org>, "leif.lindholm@linaro.org" <leif.lindholm@linaro.org>, "shannon.zhao@linaro.org" <shannon.zhao@linaro.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>, "daniel.kiper@oracle.com" <daniel.kiper@oracle.com>, "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>, "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>, "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, "linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org> Subject: Re: [PATCH] efi/libstub/fdt: Standardize the names of EFI stub parameters Message-ID: <55F1B89802000078000A1C9B@prv-mh.provo.novell.com> In-Reply-To: <20150910145331.GJ29293@leverpostej> References: <1441874516-11364-1-git-send-email-zhaoshenglong@huawei.com> <20150910095208.GA29293@leverpostej> <alpine.DEB.2.02.1509101116580.2672@kaball.uk.xensource.com> <20150910112418.GC29293@leverpostej> <alpine.DEB.2.02.1509101223580.2672@kaball.uk.xensource.com> <55F199DD02000078000A1B1E@prv-mh.provo.novell.com> <20150910145331.GJ29293@leverpostej>
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>>> On 10.09.15 at 16:53, <mark.rutland@arm.com> wrote: > On Thu, Sep 10, 2015 at 01:55:25PM +0100, Jan Beulich wrote: >> >>> On 10.09.15 at 13:37, <stefano.stabellini@eu.citrix.com> 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
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