From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brian Gerst Subject: Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure Date: Tue, 17 Oct 2017 09:10:03 -0400 Message-ID: References: <39743c79546ede3073586403d0836a4f93519b0a.1507128293.git.jpoimboe@redhat.com> <3b9fd404-6912-3b58-db29-36202631b438@oracle.com> <20171006143259.rs3zh7k5tmsgesqy@treble> <5a49e43a-8d6b-512a-ec5a-641be7bae41d@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Boris Ostrovsky Cc: Juergen Gross , Mike Galbraith , "H. Peter Anvin" , Peter Zijlstra , Andrew Cooper , Rusty Russell , Linux Virtualization , the arch/x86 maintainers , Linux Kernel Mailing List , Chris Wright , live-patching@vger.kernel.org, Linus Torvalds , Borislav Petkov , Andy Lutomirski , Josh Poimboeuf , Ingo Molnar , xen-devel@lists.xenproject.org, Thomas Gleixner , Sasha Levin , Jiri Slaby , Alok Kataria List-Id: virtualization@lists.linuxfoundation.org On Mon, Oct 16, 2017 at 2:18 PM, Boris Ostrovsky wrote: > On 10/12/2017 03:53 PM, Boris Ostrovsky wrote: >> On 10/12/2017 03:27 PM, Andrew Cooper wrote: >>> On 12/10/17 20:11, Boris Ostrovsky wrote: >>>> There is also another problem: >>>> >>>> [ 1.312425] general protection fault: 0000 [#1] SMP >>>> [ 1.312901] Modules linked in: >>>> [ 1.313389] CPU: 0 PID: 1 Comm: init Not tainted 4.14.0-rc4+ #6 >>>> [ 1.313878] task: ffff88003e2c0000 task.stack: ffffc9000038c000 >>>> [ 1.314360] RIP: 10000e030:entry_SYSCALL_64_fastpath+0x1/0xa5 >>>> [ 1.314854] RSP: e02b:ffffc9000038ff50 EFLAGS: 00010046 >>>> [ 1.315336] RAX: 000000000000000c RBX: 000055f550168040 RCX: >>>> 00007fcfc959f59a >>>> [ 1.315827] RDX: 0000000000000000 RSI: 0000000000000000 RDI: >>>> 0000000000000000 >>>> [ 1.316315] RBP: 000000000000000a R08: 000000000000037f R09: >>>> 0000000000000064 >>>> [ 1.316805] R10: 000000001f89cbf5 R11: ffff88003e2c0000 R12: >>>> 00007fcfc958ad60 >>>> [ 1.317300] R13: 0000000000000000 R14: 000055f550185954 R15: >>>> 0000000000001000 >>>> [ 1.317801] FS: 0000000000000000(0000) GS:ffff88003f800000(0000) >>>> knlGS:0000000000000000 >>>> [ 1.318267] CS: e033 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> [ 1.318750] CR2: 00007fcfc97ab218 CR3: 000000003c88e000 CR4: >>>> 0000000000042660 >>>> [ 1.319235] Call Trace: >>>> [ 1.319700] Code: 51 50 57 56 52 51 6a da 41 50 41 51 41 52 41 53 48 >>>> 83 ec 30 65 4c 8b 1c 25 c0 d2 00 00 41 f7 03 df 39 08 90 0f 85 a5 00 00 >>>> 00 50 15 9c 95 d0 ff 58 48 3d 4c 01 00 00 77 0f 4c 89 d1 ff 14 c5 >>>> [ 1.321161] RIP: entry_SYSCALL_64_fastpath+0x1/0xa5 RSP: ffffc9000038ff50 >>>> [ 1.344255] ---[ end trace d7cb8cd6cd7c294c ]--- >>>> [ 1.345009] Kernel panic - not syncing: Attempted to kill init! >>>> exitcode=0x0000000b >>>> >>>> >>>> All code >>>> ======== >>>> 0: 51 push %rcx >>>> 1: 50 push %rax >>>> 2: 57 push %rdi >>>> 3: 56 push %rsi >>>> 4: 52 push %rdx >>>> 5: 51 push %rcx >>>> 6: 6a da pushq $0xffffffffffffffda >>>> 8: 41 50 push %r8 >>>> a: 41 51 push %r9 >>>> c: 41 52 push %r10 >>>> e: 41 53 push %r11 >>>> 10: 48 83 ec 30 sub $0x30,%rsp >>>> 14: 65 4c 8b 1c 25 c0 d2 mov %gs:0xd2c0,%r11 >>>> 1b: 00 00 >>>> 1d: 41 f7 03 df 39 08 90 testl $0x900839df,(%r11) >>>> 24: 0f 85 a5 00 00 00 jne 0xcf >>>> 2a: 50 push %rax >>>> 2b:* ff 15 9c 95 d0 ff callq *-0x2f6a64(%rip) # >>>> 0xffffffffffd095cd <-- trapping instruction >>>> 31: 58 pop %rax >>>> 32: 48 3d 4c 01 00 00 cmp $0x14c,%rax >>>> 38: 77 0f ja 0x49 >>>> 3a: 4c 89 d1 mov %r10,%rcx >>>> 3d: ff .byte 0xff >>>> 3e: 14 c5 adc $0xc5,%al >>>> >>>> >>>> so the original 'cli' was replaced with the pv call but to me the offset >>>> looks a bit off, no? Shouldn't it always be positive? >>> callq takes a 32bit signed displacement, so jumping back by up to 2G is >>> perfectly legitimate. >> Yes, but >> >> ostr@workbase> nm vmlinux | grep entry_SYSCALL_64_fastpath >> ffffffff817365dd t entry_SYSCALL_64_fastpath >> ostr@workbase> nm vmlinux | grep " pv_irq_ops" >> ffffffff81c2dbc0 D pv_irq_ops >> ostr@workbase> >> >> so pv_irq_ops.irq_disable is about 5MB ahead of where we are now. (I >> didn't mean that x86 instruction set doesn't allow negative >> displacement, I was trying to say that pv_irq_ops always live further down) > > I believe the problem is this: > > #define PV_INDIRECT(addr) *addr(%rip) > > The displacement that the linker computes will be relative to the where > this instruction is placed at the time of linking, which is in > .pv_altinstructions (and not .text). So when we copy it into .text the > displacement becomes bogus. > > Replacing the macro with > > #define PV_INDIRECT(addr) *addr // well, it's not so much > indirect anymore > > makes things work. Or maybe it can be adjusted top be kept truly indirect. That is still an indirect call, just using absolute addressing for the pointer instead of RIP-relative. Alternatives has very limited relocation capabilities. It will only handle a single call or jmp replacement. Using absolute addressing is slightly less efficient (takes one extra byte to encode, and needs a relocation for KASLR), but it works just as well. You could also relocate the instruction manually by adding the delta between the original and replacement code to the displacement. -- Brian Gerst