>>> On 10.09.15 at 16:53, <mark.rutland@xxxxxxx> wrote: > On Thu, Sep 10, 2015 at 01:55:25PM +0100, Jan Beulich wrote: >> >>> On 10.09.15 at 13:37, <stefano.stabellini@xxxxxxxxxxxxx> 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? >> >> >> >> 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... >> > >> > 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. >> > >> > BTW I think that the idea you proposed was actually considered at the >> > time and deemed hard to implement, if I recall correctly. >> >> 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. > > 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. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
