Re: [PATCH v7 3/7] LoongArch: KVM: Add cpucfg area for kvm hypervisor

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On 2024/3/24 上午3:02, WANG Xuerui wrote:
On 3/15/24 16:07, Bibo Mao wrote:
Instruction cpucfg can be used to get processor features. And there
is trap exception when it is executed in VM mode, and also it is
to provide cpu features to VM. On real hardware cpucfg area 0 - 20
is used.  Here one specified area 0x40000000 -- 0x400000ff is used
for KVM hypervisor to privide PV features, and the area can be extended
for other hypervisors in future. This area will never be used for
real HW, it is only used by software.

Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
  arch/loongarch/include/asm/inst.h      |  1 +
  arch/loongarch/include/asm/loongarch.h | 10 +++++
  arch/loongarch/kvm/exit.c              | 59 +++++++++++++++++++-------
  3 files changed, 54 insertions(+), 16 deletions(-)

Sorry for the late reply, but I think it may be a bit non-constructive to repeatedly submit the same code without due explanation in our previous review threads. Let me try to recollect some of the details though...
Because your review comments about hypercall method is wrong, I need not adopt it.

If I remember correctly, during the previous reviews, it was mentioned that the only upsides of using CPUCFG were:

- it was exactly identical to the x86 approach,
- it would not require access to the LoongArch Reference Manual Volume 3 to use, and
- it was plain old data.

But, for the first point, we don't have to follow x86 convention after
X86 virtualization is successfully and widely applied in our life and products. It it normal to follow it if there is not obvious issues.

all. The second reason might be compelling, but on the one hand that's another problem orthogonal to the current one, and on the other hand HVCL is:

- already effectively public because of the fact that this very patchset is public, - its semantics is trivial to implement even without access to the LVZ manual, because of its striking similarity with SYSCALL, and - by being a function call, we reserve the possibility for hypervisors to invoke logic for self-identification purposes, even if this is likely overkill from today's perspective.

And, even if we decide that using HVCL for self-identification is overkill after all, we still have another choice that's IOCSR. We already read LOONGARCH_IOCSR_FEATURES (0x8) for its bit 11 (IOCSRF_VM) to populate the CPU_FEATURE_HYPERVISOR bit, and it's only natural that we put the identification word in the IOCSR space. As far as I can see, the IOCSR space is plenty and equally available for making reservations; it can only be even easier when it's done by a Loongson team.
IOCSR method is possible also, about chip design CPUCFG is used for cpu features and IOCSR is for device featurs. Here CPUCFG method is selected, I am KVM LoongArch maintainer and I can decide to select methods if the method works well. Is that right?

If you are interested in KVM LoongArch, you can submit more patches and become maintainer or write new hypervisor support such xen/xvisor etc, and use your method.

Also you are interested in Linux kernel, there are some issues. Can you help to improve it?

1. T0-T7 are scratch registers during SYSCALL ABI, this is what you suggest, does there exist information leaking to user space from T0-T7 registers?

2. LoongArch KVM depends on AS_HAS_LVZ_EXTENSION, which requires the latest binutils. It is also what you suggest. Some kernel developers does not have the latest binutils and common kvm code is modified and LoongArch KVM fails to compile. But they can not find it since their LoongArch cross-compile is old and LoongArch KVM is disabled. This issue can be found at

Bibo Mao

Finally, I've mentioned multiple times, that varying CPUCFG behavior based on PLV is not something well documented on the manuals, hence not friendly to low-level developers. Devs of third-party firmware and/or kernels do exist, I've personally spoken to some of them on the 2023-11-18 3A6000 release event; in order for the varying CPUCFG behavior approach to pass for me, at the very least, the LoongArch reference manual must be amended to explicitly include an explanation of it, and a reference to potential use cases.

[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux