Re: [RFC 13/16] x86/kvmclock: Share hvclock memory with the host

[Vitaly Kuznetsov]

"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:

> On Mon, May 25, 2020 at 05:22:10PM +0200, Vitaly Kuznetsov wrote:
>> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:
>> 
>> > hvclock is shared between the guest and the hypervisor. It has to be
>> > accessible by host.
>> >
>> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> > ---
>> >  arch/x86/kernel/kvmclock.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
>> > index 34b18f6eeb2c..ac6c2abe0d0f 100644
>> > --- a/arch/x86/kernel/kvmclock.c
>> > +++ b/arch/x86/kernel/kvmclock.c
>> > @@ -253,7 +253,7 @@ static void __init kvmclock_init_mem(void)
>> >  	 * hvclock is shared between the guest and the hypervisor, must
>> >  	 * be mapped decrypted.
>> >  	 */
>> > -	if (sev_active()) {
>> > +	if (sev_active() || kvm_mem_protected()) {
>> >  		r = set_memory_decrypted((unsigned long) hvclock_mem,
>> >  					 1UL << order);
>> >  		if (r) {
>> 
>> Sorry if I missed something but we have other structures which KVM guest
>> share with the host,
>> 
>> sev_map_percpu_data():
>> ...
>> 	for_each_possible_cpu(cpu) {
>> 		__set_percpu_decrypted(&per_cpu(apf_reason, cpu), sizeof(apf_reason));
>> 		__set_percpu_decrypted(&per_cpu(steal_time, cpu), sizeof(steal_time));
>> 		__set_percpu_decrypted(&per_cpu(kvm_apic_eoi, cpu), sizeof(kvm_apic_eoi));
>> 	}
>> ...
>> 
>> Do you handle them somehow in the patchset? (I'm probably just blind
>> failing to see how 'early_set_memory_decrypted()' is wired up)
>
> I don't handle them yet: I've seen the function, but have not modified it.
> I want to understand first why it doesn't blow up for me without the
> change. Any clues?

(if I got the idea of the patchset right) these features are kernel-only
(e.g. QEMU doesn't need to access these areas). E.g. for APF KVM will do
kvm_write_guest_cached() and this will use FOLL_KVM. Guests should not
rely on that and mark all shared areas as unprotected.

-- 
Vitaly