Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

From: Marc Zyngier <[email-masked]>
Date: Tue, 25 Jan 2022 15:15:54 +0000

On Wed, 19 Jan 2022 00:07:44 +0000,
Sean Christopherson <[email-masked]> wrote:
> 
> On Fri, Jan 14, 2022, Reiji Watanabe wrote:
> > 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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