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Date:   Fri, 5 May 2023 09:44:52 -0700
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Subject: Re: [PATCH v1 4/9] KVM: x86: Add new hypercall to set EPT permissions
From:   Sean Christopherson <seanjc@google.com>
To:     "=?iso-8859-1?Q?Micka=EBl_Sala=FCn?=" <mic@digikod.net>
Cc:     Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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On Fri, May 05, 2023, Micka=EF=BF=BDl Sala=EF=BF=BDn wrote:
> Add a new KVM_HC_LOCK_MEM_PAGE_RANGES hypercall that enables a guest to
> set EPT permissions on a set of page ranges.

IMO, manipulation of protections, both for memory (this patch) and CPU stat=
e
(control registers in the next patch) should come from userspace.  I have n=
o
objection to KVM providing plumbing if necessary, but I think userspace nee=
ds to
to have full control over the actual state.

One of the things that caused Intel's control register pinning series to st=
all
out was how to handle edge cases like kexec() and reboot.  Deferring to use=
rspace
means the kernel doesn't need to define policy, e.g. when to unprotect memo=
ry,
and avoids questions like "should userspace be able to overwrite pinned con=
trol
registers".

And like the confidential VM use case, keeping userspace in the loop is a b=
ig
beneifit, e.g. the guest can't circumvent protections by coercing userspace=
 into
writing to protected memory .