On Wed, 19 Jan 2022 00:07:44 +0000,
Sean Christopherson [off-list ref] wrote:

On Fri, Jan 14, 2022, Reiji Watanabe wrote:

quoted

The restriction, with which KVM doesn't need to worry about the changes
in the registers after KVM_RUN, could potentially protect or be useful
to protect KVM and simplify future changes/maintenance of the KVM codes
that consumes the values.

That sort of protection is definitely welcome, the previously mentioned CPUID mess
on x86 would have benefit greatly by KVM being restrictive in the past.  That said,
hooking KVM_RUN is likely the wrong way to go about implementing any restrictions.
Running a vCPU is where much of the vCPU's state is explicitly consumed, but it's
all too easy for KVM to implicity/indirectly consume state via a different ioctl(),
e.g. if there are side effects that are visible in other registers, than an update
can also be visible to userspace via KVM_{G,S}ET_{S,}REGS, at which point disallowing
modifying state after KVM_RUN but not after reading/writing regs is arbitrary and
inconsitent.

If possible, preventing modification if kvm->created_vcpus > 0 is
ideal as it's a relatively common pattern in KVM, and provides a
clear boundary to userpace regarding what is/isn't allowed.

No, that's way too late. The configuration is in general per-CPU, and
I really don't want to expand the surface of the userspace API to
allow all sort of magic trick depending on the nature of what you
save/restore.

The "first run" crap is already there. We have it on a per-CPU basis,
and we need it at the VM level for other reasons (see the recent
discussion about PMU filtering vs binding to a specific PMU
implementation).

	M.

-- 
Without deviation from the norm, progress is not possible.

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