Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state

[Dave Hansen]

On 5/17/21 12:46 AM, Paolo Bonzini wrote:
> On 14/05/21 07:11, Andy Lutomirski wrote:
>> That's nice, but it fails to restore XINUSE[PKRU].  As far as I know,
>> that bit is live, and the only way to restore it to 0 is with
>> XRSTOR(S).
> 
> The manual says "It is possible for XINUSE[i] to be 1 even when state
> component i is in its initial configuration" so this is architecturally
> valid.  Does the XINUSE optimization matter for PKRU which is a single
> word?

In Linux with normal userspace, virtually never.

The hardware defaults PKRU to 0x0 which means "no restrictions on any
keys".  Linux defaults PKRU via 'init_pkru_value' to the most
restrictive value.  This ensures that new non-zero-pkey-assigned memory
is protected by default.

But, that also means PKRU is virtually never in its init state in Linux.
 An app would probably need to manipulate PKRU with XRSTOR to get
XINUSE[PKRU]=0.

It would only even *possibly* be useful if running a KVM guest that had
PKRU=0x0 (sorry I don't consider things using KVM "normal userspace" :P ).