Re: [PATCH v4 0/9] x86/sev: KEXEC/KDUMP support for SEV-ES guests

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/11/24 11:17, Vasant Karasulli wrote:
> From: Vasant Karasulli <vkarasulli@suse.de>
> 
> Hi,

Hi Vasant,

The SNP guest support has been incorporated in the kernel since this 
patchset was originally presented. SNP also is considered a guest with 
encrypted state (CC_ATTR_GUEST_STATE_ENCRYPT will return true), but does 
not use the AP jump table. So this series need adjusted so that the AP 
jump table is only used for SEV-ES guests.

Thanks,
Tom

> 
> 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 VMGEXIT,
> 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.
> 
> The previous version of this patch-set can be found here:
> 
> 	https://lore.kernel.org/lkml/20220127101044.13803-1-joro@8bytes.org/
> 
> Please review.
> 
> Thanks,
>     Vasant
> 
> Changes v3->v4:
>          - Rebased to v6.8 kernel
> 	- Applied review comments by Sean Christopherson
> 	- Combined sev_es_setup_ap_jump_table() and sev_setup_ap_jump_table()
>            into a single function which makes caching jump table address
>            unnecessary
>          - annotated struct sev_ap_jump_table_header with __packed attribute
> 	- added code to set up real mode data segment at boot time instead of
>            hardcoding the value.
> 
> Changes v2->v3:
> 
> 	- Rebased to v5.17-rc1
> 	- Applied most review comments by Boris
> 	- Use the name 'AP jump table' consistently
> 	- Make kexec-disabling for unsupported guests x86-specific
> 	- Cleanup and consolidate patches to detect GHCB v2 protocol
> 	  support
> 
> Joerg Roedel (9):
>    x86/kexec/64: Disable kexec when SEV-ES is active
>    x86/sev: Save and print negotiated GHCB protocol version
>    x86/sev: Set GHCB data structure version
>    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/kexec/64: Support kexec under SEV-ES with AP Jump Table Blob
> 
>   arch/x86/boot/compressed/sev.c          |  45 +-
>   arch/x86/include/asm/insn-eval.h        |   1 +
>   arch/x86/include/asm/realmode.h         |   5 +
>   arch/x86/include/asm/sev-ap-jumptable.h |  30 +
>   arch/x86/include/asm/sev.h              |   7 +
>   arch/x86/kernel/machine_kexec_64.c      |  12 +
>   arch/x86/kernel/process.c               |   8 +
>   arch/x86/kernel/sev-shared.c            | 234 +++++-
>   arch/x86/kernel/sev.c                   | 372 +++++-----
>   arch/x86/lib/insn-eval-shared.c         | 912 ++++++++++++++++++++++++
>   arch/x86/lib/insn-eval.c                | 911 +----------------------
>   arch/x86/realmode/Makefile              |   9 +-
>   arch/x86/realmode/rm/Makefile           |  11 +-
>   arch/x86/realmode/rm/header.S           |   3 +
>   arch/x86/realmode/rm/sev.S              |  85 +++
>   arch/x86/realmode/rmpiggy.S             |   6 +
>   arch/x86/realmode/sev/Makefile          |  33 +
>   arch/x86/realmode/sev/ap_jump_table.S   | 131 ++++
>   arch/x86/realmode/sev/ap_jump_table.lds |  24 +
>   19 files changed, 1695 insertions(+), 1144 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.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: e8f897f4afef0031fe618a8e94127a0934896aba
> --
> 2.34.1
>