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Re: [Xen-devel] Xen 4.1 + Linux compiled with PVH == BOOM

On Thu, Jan 02, 2014 at 03:30:52PM -0400, Konrad Rzeszutek Wilk wrote: > On Mon, Dec 30, 2013 at 02:56:48PM -0500, Konrad Rzeszutek Wilk wrote: > > On Tue, Dec 24, 2013 at 01:05:10PM +0000, David Vrabel wrote: > > > On 24/12/2013 12:55, Andrew Cooper wrote: > > > > On 24/12/2013 12:31, David Vrabel wrote: > > > >> On 20/12/2013 17:57, Konrad Rzeszutek Wilk wrote: > > > >>> Hey, > > > >>> > > > >>> This is with Linux and > > > >>> > > > >>> git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git > > > >>> stable/pvh.v11 > > > >>> > > > >>> I get Xen 4.1 (only) hypervisor to blow up with a Linux kernel that > > > >>> has been > > > >>> compiled with PVH. > > > >>> > > > >>> I think the same problem would show up if I tried to launch a PV > > > >>> guest > > > >>> compiled as PVH under Xen 4.1 as well - as the ELF parsing code is > > > >>> shared > > > >>> with the toolstack. > > > >> If a kernel with both PVH and PV support enabled cannot boot in PV mode > > > >> with a non-PVH aware hypervisor/toolstack then the kernel is broken. > > > >> > > > >> Hypervisor/tool-side fixes aren't the correct fix here. Xen 4.1 and > > > >> even older are still widely deployed. > > > >> > > > >> David > > > > > > > > I believe that the problem is because the elf parsing code is not > > > > sufficiently forward-compatible aware, and rejects the PVH kernel > > > > because it has an unrecognised Xen elf note field. This is not a kernel > > > > bug. > > > > It (Xen 4.1) has the logic to ignore unrecognized Xen elf note fields. But > > it (all Xen versions) do not have the logic to ignore in the > > "SUPPORTED_FEATURES" > > an unrecognized string. > > > > > > > > > > The elf parsing should accept unrecognised fields for forward > > > > compatibility, which would then allow a PV & PVH compiled kernel to run > > > > in PV mode. > > > > > > It should but it doesn't, so a different way needs to be found for the > > > kernel to report (optional) PVH support. A method that is compatible > > > with older toolstacks. > > > > Also known as changes to the PVH ABI. > > > > Mukesh, Roger, George (emailing Ian instead since he is now the Release > > Manager-pro-temp), Jan, > > > > a). That means dropping the 'hvm_callback_vector' check from xc_dom_core.c > > and > > just depending on: > > "writable_descriptor_tables|auto_translated_physmap|supervisor_mode_kernel" > > for PVH guests. > > > > b) Or dropping that altogether and introducing a new Xen elf note field, > > say: > > > > XEN_ELFNOTE_PVH_VERSION > > > > c). > > Use the 'XEN_ELFNOTE_SUPPORTED_FEATURES' which says: > * > * Other than XEN_ELFNOTE_FEATURES on pre-4.2 Xen, this note allows a > * kernel to specify support for features that older hypervisors don't > * know about. The set of features 4.2 and newer hypervisors will > * consider supported by the kernel is the combination of the sets > * specified through this and the string note. > > for hvm_callback_vector parameter. > > > > > Which way should we do this? > > The c) way looks the best. Ian, would you be OK with that idea for 4.4? Seems that not only does it work without any changes in Xen 4.4 but it is all in the Linux kernel, and it allows us to boot an Linux kernel with PV and PVH support Seems that not only does it work without any changes in Xen 4.4 but it is all in the Linux kernel: diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S index 56f42c0..2ce56bf 100644 --- a/arch/x86/xen/xen-head.S +++ b/arch/x86/xen/xen-head.S @@ -11,12 +11,22 @@ #include <asm/page_types.h> #include <xen/interface/elfnote.h> +#include <xen/interface/features.h> #include <asm/xen/interface.h> #ifdef CONFIG_XEN_PVH -#define PVH_FEATURES_STR "|writable_descriptor_tables|auto_translated_physmap|supervisor_mode_kernel|hvm_callback_vector" +#define PVH_FEATURES_STR "|writable_descriptor_tables|auto_translated_physmap|supervisor_mode_kernel" +/* Note the lack of 'hvm_callback_vector'. Older hypervisor will + * balk at this being part of XEN_ELFNOTE_FEATURES, so we put it in + * XEN_ELFNOTE_SUPPORTED_FEATURES which older hypervisors will ignore. + */ +#define PVH_FEATURES ((1 << XENFEAT_writable_page_tables) | \ + (1 << XENFEAT_auto_translated_physmap) | \ + (1 << XENFEAT_supervisor_mode_kernel) | \ + (1 << XENFEAT_hvm_callback_vector)) #else #define PVH_FEATURES_STR "" +#define PVH_FEATURES (0) #endif __INIT @@ -102,6 +112,9 @@ NEXT_HYPERCALL(arch_6) ELFNOTE(Xen, XEN_ELFNOTE_ENTRY, _ASM_PTR startup_xen) ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, _ASM_PTR hypercall_page) ELFNOTE(Xen, XEN_ELFNOTE_FEATURES, .ascii "!writable_page_tables|pae_pgdir_above_4gb"; .asciz PVH_FEATURES_STR) + ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES, .long (PVH_FEATURES) | + (1 << XENFEAT_writable_page_tables) | + (1 << XENFEAT_dom0)) ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE, .asciz "yes") ELFNOTE(Xen, XEN_ELFNOTE_LOADER, .asciz "generic") ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID, diff --git a/include/xen/interface/elfnote.h b/include/xen/interface/elfnote.h index 0360b15..6f4eae3 100644 --- a/include/xen/interface/elfnote.h +++ b/include/xen/interface/elfnote.h @@ -140,6 +140,19 @@ */ #define XEN_ELFNOTE_SUSPEND_CANCEL 14 +/* + * The features supported by this kernel (numeric). + * + * Other than XEN_ELFNOTE_FEATURES on pre-4.2 Xen, this note allows a + * kernel to specify support for features that older hypervisors don't + * know about. The set of features 4.2 and newer hypervisors will + * consider supported by the kernel is the combination of the sets + * specified through this and the string note. + * + * LEGACY: FEATURES + */ +#define XEN_ELFNOTE_SUPPORTED_FEATURES 17 + #endif /* __XEN_PUBLIC_ELFNOTE_H__ */ /* _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel

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