Re: Re: [PATCH] KVM:arm/arm64: dcache need be coherent unconditionally

On 09/03/18 22:15, Peng Hao wrote:

quoted

For emulation devices just like vga, keeping coherent dcache between
guest and host timely is needed.
Now the display of vnc-viewer will not update continuously and the
patch can fix up.

Signed-off-by: Peng Hao <redacted>
---
 virt/kvm/arm/mmu.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index ec62d1c..4a28395e 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c

@@ -1416,8 +1416,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
             kvm_set_pfn_dirty(pfn);
         }
 
-        if (fault_status != FSC_PERM)
-            clean_dcache_guest_page(pfn, PMD_SIZE);
+        clean_dcache_guest_page(pfn, PMD_SIZE);
 
         if (exec_fault) {
             new_pmd = kvm_s2pmd_mkexec(new_pmd);

@@ -1438,8 +1437,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
             mark_page_dirty(kvm, gfn);
         }
 
-        if (fault_status != FSC_PERM)
-            clean_dcache_guest_page(pfn, PAGE_SIZE);
+        clean_dcache_guest_page(pfn, PAGE_SIZE);
 
         if (exec_fault) {
             new_pte = kvm_s2pte_mkexec(new_pte);

I'm sorry, but I have to NAK this.

You're papering over the fundamental issue that you're accessing a
cacheable alias of a non cacheable memory. The architecture is very
clear about why this doesn't work, and KVM implements the architecture.

I find that I  just encounter the problem after  the commit '15303ba5d1cd9b28d03a980456c0978c0ea3b208 " .
The commit contains "icache invalidation optimizations, improving VM startup time",it changes
from unconditionally calling coherent_cache_guest_page(including dcache handle) to conditionally calling clean_dcache_guest_page.
 I trace the display of vnc abnormally and find it generate data abort in vga
address region with FSC_PERM,and it will not call clean_dcache_guest_page . So I think  should recover to uncontionally calling clean_dcache_guest_page.

Thanks.

If you want this to work, map your VGA device as cacheable, add cache
maintenance to QEMU, or use another frame-buffer emulation that doesn't
require such a gack.

Thanks,

   M.
-- 
Jazz is not dead. It just smells funny...