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Re: [PATCH v7 1/4] tcg: add 'size' param to probe_access_flags()

From: Richard Henderson
Subject: Re: [PATCH v7 1/4] tcg: add 'size' param to probe_access_flags()
Date: Thu, 23 Feb 2023 14:06:42 -1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 2/23/23 13:44, Daniel Henrique Barboza wrote:
probe_access_flags() as it is today uses probe_access_full(), which in
turn uses probe_access_internal() with size = 0. probe_access_internal()
then uses the size to call the tlb_fill() callback for the given CPU.
This size param ('fault_size' as probe_access_internal() calls it) is
ignored by most existing .tlb_fill callback implementations, e.g.
arm_cpu_tlb_fill(), ppc_cpu_tlb_fill(), x86_cpu_tlb_fill() and
mips_cpu_tlb_fill() to name a few.

But RISC-V riscv_cpu_tlb_fill() actually uses it. The 'size' parameter
is used to check for PMP (Physical Memory Protection) access. This is
necessary because PMP does not make any guarantees about all the bytes
of the same page having the same permissions, i.e. the same page can
have different PMP properties, so we're forced to make sub-page range
checks. To allow RISC-V emulation to do a probe_acess_flags() that
covers PMP, we need to either add a 'size' param to the existing
probe_acess_flags() or create a new interface (e.g.

There are quite a few probe_* APIs already, so let's add a 'size' param
to probe_access_flags() and re-use this API. This is done by open coding
what probe_access_full() does inside probe_acess_flags() and passing the
'size' param to probe_acess_internal(). Existing probe_access_flags()
callers use size = 0 to not change their current API usage. 'size' is
asserted to enforce single page access like probe_access() already does.

No behavioral changes intended.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
  accel/stubs/tcg-stub.c        |  2 +-
  accel/tcg/cputlb.c            | 17 ++++++++++++++---
  accel/tcg/user-exec.c         |  5 +++--
  include/exec/exec-all.h       |  3 ++-
  semihosting/uaccess.c         |  2 +-
  target/arm/ptw.c              |  2 +-
  target/arm/sve_helper.c       |  2 +-
  target/s390x/tcg/mem_helper.c |  6 +++---
  8 files changed, 26 insertions(+), 13 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

I'll change probe_access_full as well, just to keep them in sync.

+int probe_access_flags(CPUArchState *env, target_ulong addr, int size,
                         MMUAccessType access_type, int mmu_idx,
                         bool nonfault, void **phost, uintptr_t retaddr)
      CPUTLBEntryFull *full;
+    int flags;
+    g_assert(-(addr | TARGET_PAGE_MASK) >= size);
+    flags = probe_access_internal(env, addr, size, access_type, mmu_idx,
+                                  nonfault, phost, &full, retaddr);
- return probe_access_full(env, addr, access_type, mmu_idx,
-                             nonfault, phost, &full, retaddr);
+    /* Handle clean RAM pages. */
+    if (unlikely(flags & TLB_NOTDIRTY)) {
+        notdirty_write(env_cpu(env), addr, 1, full, retaddr);
+        flags &= ~TLB_NOTDIRTY;
+    }
+    return flags;

But bypassing probe_access_full here is fine.


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