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From:   Joerg Roedel <joro@8bytes.org>
To:     x86@kernel.org, Eric Biederman <ebiederm@xmission.com>
Cc:     kexec@lists.infradead.org, Joerg Roedel <jroedel@suse.de>,
        hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>,
        Kees Cook <keescook@chromium.org>,
        David Rientjes <rientjes@google.com>,
        Cfir Cohen <cfir@google.com>,
        Erdem Aktas <erdemaktas@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mike Stunes <mstunes@vmware.com>,
        Sean Christopherson <seanjc@google.com>,
        Martin Radev <martin.b.radev@gmail.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Joerg Roedel <joro@8bytes.org>, linux-coco@lists.linux.dev,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: [PATCH 00/12] x86/sev: KEXEC/KDUMP support for SEV-ES guests
Date:   Wed, 21 Jul 2021 16:20:03 +0200
Message-Id: <20210721142015.1401-1-joro@8bytes.org>
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From: Joerg Roedel <jroedel@suse.de>

Hi,

here are changes to enable kexec/kdump in SEV-ES guests. The biggest
problem for supporting kexec/kdump under SEV-ES is to find a way to
hand the non-boot CPUs (APs) from one kernel to another.

Without SEV-ES the first kernel parks the CPUs in a HLT loop until
they get reset by the kexec'ed kernel via an INIT-SIPI-SIPI sequence.
For virtual machines the CPU reset is emulated by the hypervisor,
which sets the vCPU registers back to reset state.

This does not work under SEV-ES, because the hypervisor has no access
to the vCPU registers and can't make modifications to them. So an
SEV-ES guest needs to reset the vCPU itself and park it using the
AP-reset-hold protocol. Upon wakeup the guest needs to jump to
real-mode and to the reset-vector configured in the AP-Jump-Table.

The code to do this is the main part of this patch-set. It works by
placing code on the AP Jump-Table page itself to park the vCPU and for
jumping to the reset vector upon wakeup. The code on the AP Jump Table
runs in 16-bit protected mode with segment base set to the beginning
of the page. The AP Jump-Table is usually not within the first 1MB of
memory, so the code can't run in real-mode.

The AP Jump-Table is the best place to put the parking code, because
the memory is owned, but read-only by the firmware and writeable by
the OS. Only the first 4 bytes are used for the reset-vector, leaving
the rest of the page for code/data/stack to park a vCPU. The code
can't be in kernel memory because by the time the vCPU wakes up the
memory will be owned by the new kernel, which might have overwritten it
already.

The other patches add initial GHCB Version 2 protocol support, because
kexec/kdump need the MSR-based (without a GHCB) AP-reset-hold VMGEXT,
which is a GHCB protocol version 2 feature.

The kexec'ed kernel is also entered via the decompressor and needs
MMIO support there, so this patch-set also adds MMIO #VC support to
the decompressor and support for handling CLFLUSH instructions.

Finally there is also code to disable kexec/kdump support at runtime
when the environment does not support it (e.g. no GHCB protocol
version 2 support or AP Jump Table over 4GB).

The diffstat looks big, but most of it is moving code for MMIO #VC
support around to make it available to the decompressor.

Known problems:

	It currently only works reliable when the kexec_file syscall
	is used. With the kexec syscall it works one time, and the
	second kexec then hangs because the sha256 verification fails
	in the purgatory.

	For the kexec syscall the purgatory is loaded from user-space,
	were most likely the issue needs to be fixed. But the question
	is whether this is a useful way. The user-space purgatory has
	other ways to break under SEV-ES, which can be enabled via
	kexec-tool command line options. For example enabling console
	support for the purgatory will definitly break kexec for
	SEV-ES.

	So the question is whether is makes sense to only support
	kexec_file() under SEV-ES, where the in-kernel purgatory is
	used (which has less ways to break under SEV-ES).

With that said, please review :)

Regards,

	Joerg

Joerg Roedel (12):
  kexec: Allow architecture code to opt-out at runtime
  x86/kexec/64: Forbid kexec when running as an SEV-ES guest
  x86/sev: Save and print negotiated GHCB protocol version
  x86/sev: Do not hardcode GHCB protocol version
  x86/sev: Use GHCB protocol version 2 if supported
  x86/sev: Cache AP Jump Table Address
  x86/sev: Setup code to park APs in the AP Jump Table
  x86/sev: Park APs on AP Jump Table with GHCB protocol version 2
  x86/sev: Use AP Jump Table blob to stop CPU
  x86/sev: Add MMIO handling support to boot/compressed/ code
  x86/sev: Handle CLFLUSH MMIO events
  x86/sev: Support kexec under SEV-ES with AP Jump Table blob

 arch/x86/boot/compressed/sev.c          |  56 +-
 arch/x86/include/asm/realmode.h         |   5 +
 arch/x86/include/asm/sev-ap-jumptable.h |  25 +
 arch/x86/include/asm/sev.h              |  13 +-
 arch/x86/kernel/machine_kexec_64.c      |  12 +
 arch/x86/kernel/process.c               |   8 +
 arch/x86/kernel/sev-shared.c            | 333 +++++++++-
 arch/x86/kernel/sev.c                   | 494 ++++++---------
 arch/x86/lib/insn-eval-shared.c         | 805 ++++++++++++++++++++++++
 arch/x86/lib/insn-eval.c                | 802 +----------------------
 arch/x86/realmode/Makefile              |   9 +-
 arch/x86/realmode/rm/Makefile           |  11 +-
 arch/x86/realmode/rm/header.S           |   3 +
 arch/x86/realmode/rm/sev_ap_park.S      |  89 +++
 arch/x86/realmode/rmpiggy.S             |   6 +
 arch/x86/realmode/sev/Makefile          |  41 ++
 arch/x86/realmode/sev/ap_jump_table.S   | 130 ++++
 arch/x86/realmode/sev/ap_jump_table.lds |  24 +
 include/linux/kexec.h                   |   1 +
 kernel/kexec.c                          |  14 +
 kernel/kexec_file.c                     |   9 +
 21 files changed, 1764 insertions(+), 1126 deletions(-)
 create mode 100644 arch/x86/include/asm/sev-ap-jumptable.h
 create mode 100644 arch/x86/lib/insn-eval-shared.c
 create mode 100644 arch/x86/realmode/rm/sev_ap_park.S
 create mode 100644 arch/x86/realmode/sev/Makefile
 create mode 100644 arch/x86/realmode/sev/ap_jump_table.S
 create mode 100644 arch/x86/realmode/sev/ap_jump_table.lds


base-commit: 2734d6c1b1a089fb593ef6a23d4b70903526fe0c
-- 
2.31.1