[PATCH] KVM: allow NULL writable argument to __kvm_faultin_pfn

[Paolo Bonzini]

kvm_follow_pfn() is able to work with NULL in the .map_writable field
of the homonymous struct.  But __kvm_faultin_pfn() rejects the combo
despite KVM for e500 trying to use it.  Indeed .map_writable is not
particularly useful if the flags include FOLL_WRITE and readonly
guest memory is not supported, so add support to __kvm_faultin_pfn()
for this case.

Fixes: 1c7b627e9306 ("KVM: Add kvm_faultin_pfn() to specifically service guest page faults")
Reported-by: Christian Zigotzky <chzigotzky@xxxxxxxxxxx>
Tested-by: Christian Zigotzky <chzigotzky@xxxxxxxxxxx>
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: regressions@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index de2c11dae231..5177e56fdbd5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2975,10 +2975,11 @@ kvm_pfn_t __kvm_faultin_pfn(const struct kvm_memory_slot *slot, gfn_t gfn,
 		.refcounted_page = refcounted_page,
 	};
 
-	if (WARN_ON_ONCE(!writable || !refcounted_page))
+	if (WARN_ON_ONCE(!refcounted_page))
 		return KVM_PFN_ERR_FAULT;
 
-	*writable = false;
+	if (writable)
+		*writable = false;
 	*refcounted_page = NULL;
 
 	return kvm_follow_pfn(&kfp);
-- 
2.43.5