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[PATCH v5 0/3] target/ppc: single step for KVM HV
From: |
Fabiano Rosas |
Subject: |
[PATCH v5 0/3] target/ppc: single step for KVM HV |
Date: |
Wed, 11 Dec 2019 16:10:10 -0300 |
I'm resending this series now that the KVM capability got merged:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1a9167a214f
---
Original cover-letter:
Single stepping via GDB/gdbstub is currently not working with KVM
HV. When asking for a single step (stepi), KVM simply ignores the
request and execution continues.
This has the direct effect of breaking GDB's 'step', 'stepi', 'next',
'nexti' commands. The 'continue' command is also affected since
continuing right after a breakpoint requires that GDB first perform a
single step so that the breakpoint can be re-inserted before
continuing - in this case the breakpoint is not re-inserted and it
won't hit again.
The issue here is that single stepping in POWER makes use of an
interrupt (Trace Interrupt [1]) that does not reach the hypervisor, so
while the single step would happen if properly triggered, it would not
cause an exit to KVM so there would be no way of handing control back
to GDB. Aside from that, the guest kernel is not prepared to deal with
such an interrupt in kernel mode (when not using KGDB, or some other
debugging facility) and it causes an Oops.
This series implements a "software single step" approach that makes
use of: i) the Trace Interrupt to perform the step inside the guest
and ii) a breakpoint at the Trace Interrupt handler address to cause a
vm exit (Emulation Assist) so that we can return control to QEMU.
With (i), we basically get the single step for free, without having to
discover what are the possible targets of instructions that divert
execution.
With (ii), we hide the single step from the guest and keep all of the
step logic in QEMU.
Supported scenarios:
- Stepping of multiple vcpus;
- GDB scheduler locking on and off [2];
- single stepping of kernel code with QEMU while stepping with GDB
inside the guest (user space, KGDB).
1- PowerISA Section 6.5.15 - Trace Interrupt
2- https://sourceware.org/gdb/onlinedocs/gdb/All_002dStop-Mode.html
v4 -> v5:
- rebase to v4.2.0-rc5;
- use KVM_CAP_PPC_GUEST_DEBUG_SSTEP (#176) which is now in kernel
v5.5-rc1:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1a9167a214f
v3 -> v4:
- patch 1: fix uninitialized 'offset' variable;
- patch 2: squash with patch 7/7 (now 5/5);
fix exception vector offset calculation when AIL == 0;
- patch 3: squash with 4/7 (now 2/5);
- patch 7: introduce ppc_gdb_get_{op,xop,spr} functions;
introduce ppc_gdb_read_insn function;
define constants for mfmsr, rfid, mtspr extended opcodes;
fix bug where instructions would not be properly
recognized in SLOF due to guest endianness being different
from host's;
pass arch_info->address directly into functions that only
need the address;
improve indentation by returning early when possible.
https://lists.nongnu.org/archive/html/qemu-devel/2019-02/msg07994.html
v2 -> v3:
- take Alternate Interrupt Location (AIL) into consideration when
calculating the Trace Interrupt handler address (this allows single
stepping in SLOF code);
- check for a new KVM_GUEST_DEBUG_SSTEP capability (still to be
submitted to kernel ml);
- handle other vcpus (not currently stepping) hitting the single step
breakpoint - by ignoring the breakpoint;
- handle simultaneous single step by GDB inside guest - by first
performing our step into the trace interrupt handler itself and
returning to the guest afterwards;
- handle single stepping when at the first trace interrupt handler
instruction - by displacing the breakpoint to the next instruction;
- restore MSR, SRR0, SRR1 after the step, taking into consideration
possible mtspr, mtmsr instructions;
- use stubs for arch-specific code that will not be implemented by
other architectures at this point;
https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg04627.html
v1 -> v2:
- split in more patches to facilitate review
- use extract32 for decoding instruction instead of open-coding
- add more people to CC
https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg04269.html
v1:
https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg03738.html
Fabiano Rosas (3):
linux-headers: Update kvm.h for ppc single step capability
kvm-all: Introduce kvm_set_singlestep
target/ppc: support single stepping with KVM HV
accel/kvm/kvm-all.c | 14 +++
accel/stubs/kvm-stub.c | 4 +
exec.c | 2 +-
include/sysemu/kvm.h | 4 +
linux-headers/linux/kvm.h | 1 +
stubs/Makefile.objs | 1 +
stubs/kvm-arch-singlestep.c | 14 +++
target/ppc/cpu.h | 16 +++
target/ppc/excp_helper.c | 13 +++
target/ppc/gdbstub.c | 35 ++++++
target/ppc/kvm.c | 209 ++++++++++++++++++++++++++++++++++--
11 files changed, 305 insertions(+), 8 deletions(-)
create mode 100644 stubs/kvm-arch-singlestep.c
--
2.23.0
- [PATCH v5 0/3] target/ppc: single step for KVM HV,
Fabiano Rosas <=