Re: [PATCH v5 3/6] LoongArch: KVM: Add cpucfg area for kvm hypervisor

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On 2/27/24 11:14, maobibo wrote:

On 2024/2/27 上午4:02, Jiaxun Yang wrote:

在2024年2月26日二月 上午8:04,maobibo写道:
On 2024/2/26 下午2:12, Huacai Chen wrote:
On Mon, Feb 26, 2024 at 10:04 AM maobibo <maobibo@xxxxxxxxxxx> wrote:

On 2024/2/24 下午5:13, Huacai Chen wrote:
Hi, Bibo,

On Thu, Feb 22, 2024 at 11:28 AM Bibo Mao <maobibo@xxxxxxxxxxx> 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.
After reading and thinking, I find that the hypercall method which is
used in our productive kernel is better than this cpucfg method.
Because hypercall is more simple and straightforward, plus we don't
worry about conflicting with the real hardware.
No, I do not think so. cpucfg is simper than hypercall, hypercall can
be in effect when system runs in guest mode. In some scenario like TCG
mode, hypercall is illegal intruction, however cpucfg can work.
Nearly all architectures use hypercall except x86 for its historical
Only x86 support multiple hypervisors and there is multiple hypervisor
in x86 only. It is an advantage, not historical reason.

I do believe that all those stuff should not be exposed to guest user space
for security reasons.
Can you add PLV checking when cpucfg 0x40000000-0x400000FF is emulated? if it is user mode return value is zero and it is kernel mode emulated value will be returned. It can avoid information leaking.

I've suggested this approach in another reply [1], but I've rechecked the manual, and it turns out this behavior is not permitted by the current wording. See LoongArch Reference Manual v1.10, Volume 1, Section "CPUCFG":

> CPUCFG 访问未定义的配置字将读回全 0 值。
> Reads of undefined CPUCFG configuration words shall return all-zeroes.

This sentence mentions no distinction based on privilege modes, so it can only mean the behavior applies universally regardless of privilege modes.

I think if you want to make CPUCFG behavior PLV-dependent, you may have to ask the LoongArch spec editors, internally or in public, for a new spec revision.

(There are already multiple third-party LoongArch implementers as of late 2023, so any ISA-level change like this would best be coordinated, to minimize surprises.)


WANG "xen0n" Xuerui

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