[PATCH] KVM: arm64: Allow smaller FF-A RX/TX buffers

[Kim Mankyum via B4 Relay]

From: Kim Mankyum <mankyum.kim@xxxxxxxxxxx>

pKVM currently requires the host to map PAGE_SIZE worth of RX/TX
buffers, as do_ffa_rxtx_map() requires the FF-A page count to match
the capacity of the hypervisor RX/TX buffers.

This is unnecessarily restrictive when the kernel page size is larger
than the minimum RX/TX buffer size advertised by the SPMC. For
example, with a 16K kernel and an SPMC advertising a minimum RX/TX
buffer size of 4K, a host request with a page count of one is rejected
by pKVM.

hyp_ffa_post_init() already allows an SPMC whose minimum RX/TX buffer
size is smaller than PAGE_SIZE, and the page count supplied by the host
is forwarded unchanged when mapping the hypervisor buffers into the
SPMC.

Allow RX/TX buffers smaller than the capacity of the hypervisor buffers
while continuing to reject requests which exceed it.

Fixes: 9d0c6a9af9e3 ("KVM: arm64: Handle FFA_RXTX_MAP and FFA_RXTX_UNMAP calls from the host")
Signed-off-by: Kim Mankyum <mankyum.kim@xxxxxxxxxxx>
---
 arch/arm64/kvm/hyp/nvhe/ffa.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
index a327c2bbb6b6..594d1112b5f9 100644
--- a/arch/arm64/kvm/hyp/nvhe/ffa.c
+++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
@@ -239,7 +239,8 @@ static void do_ffa_rxtx_map(struct arm_smccc_1_2_regs *res,
 	int ret = 0;
 	void *rx_virt, *tx_virt;
 
-	if (npages != (KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE) / FFA_PAGE_SIZE) {
+	if (!npages ||
+	    npages > (KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE) / FFA_PAGE_SIZE) {
 		ret = FFA_RET_INVALID_PARAMETERS;
 		goto out;
 	}

---
base-commit: cb8a75eec0877810b50aa1c5a833f929525cd2ee
change-id: 20260820-master-572418a358ab

Best regards,
--  
Kim Mankyum <mankyum.kim@xxxxxxxxxxx>