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Re: [PATCH v3 03/19] target/arm: Restrict DC-CVAP instruction to TCG acc

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v3 03/19] target/arm: Restrict DC-CVAP instruction to TCG accel
Date: Fri, 17 Apr 2020 16:19:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 4/17/20 3:54 PM, Peter Maydell wrote:
On Fri, 17 Apr 2020 at 14:49, Philippe Mathieu-Daudé <address@hidden> wrote:

On 3/16/20 9:11 PM, Peter Maydell wrote:
On Mon, 16 Mar 2020 at 19:36, Richard Henderson
<address@hidden> wrote:
I'm not 100% sure how the system regs function under kvm.

If they are not used at all, then we should avoid them all en masse an not
piecemeal like this.

If they are used for something, then we should keep them registered and change
the writefn like so:

      /* existing stuff */
      /* Handled by hardware accelerator. */

I ended with that patch because dccvap_writefn() calls probe_read()
which is an inlined call to probe_access(), which itself is only defined
when using TCG. So with KVM either linking fails or I get:

target/arm/helper.c: In function ‘dccvap_writefn’:
target/arm/helper.c:6898:13: error: implicit declaration of function
       haddr = probe_read(env, vaddr, dline_size, mem_idx, GETPC());

IN this particular case, DC CVAP is really a system insn rather
than a 'register'; our register struct for it is marked up as
ARM_CP_NO_RAW, which means we'll effectively ignore it when
running KVM (it will not be migrated, have its state synced
against the kernel, or be visible in gdb). If dccvap_writefn()
ever gets called somehow that's a bug, so having it end up
with an assert is the right thing.

I'll use your suggestion which works for me:

Your suggested patch isn't quite the same as RTH's suggestion,
because it puts the assert inside a stub probe_read()
implementation rather than having the ifdef at the level
of the writefn body. I have no opinion on whether one or
the other of these is preferable.

I'll let Richard modify the writefn() bodies if required, as he understand what they do :)

Btw since we have this rule:

obj-$(call lnot,$(CONFIG_TCG))  += tcg-stub.o

I'll use the following patch which is less intrusive:

-- >8 --
index 677191a69c..e4bbf997aa 100644
--- a/accel/stubs/tcg-stub.c
+++ b/accel/stubs/tcg-stub.c
@@ -22,3 +22,10 @@ void tb_flush(CPUState *cpu)
 void tlb_set_dirty(CPUState *cpu, target_ulong vaddr)
+void *probe_access(CPUArchState *env, target_ulong addr, int size,
+ MMUAccessType access_type, int mmu_idx, uintptr_t retaddr)
+     /* Handled by hardware accelerator. */
+     g_assert_not_reached();

-- PMM

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