Re: [RFC][PATCH 1/2] kvm: x86: mmu: return zero if s > e in rsvd_bits()

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On 2014/11/14 18:06, Paolo Bonzini wrote:


On 14/11/2014 10:31, Tiejun Chen wrote:
In some real scenarios 'start' may not be less than 'end' like
maxphyaddr = 52.

Signed-off-by: Tiejun Chen <tiejun.chen@xxxxxxxxx>
---
  arch/x86/kvm/mmu.h | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index bde8ee7..0e98b5e 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -58,6 +58,8 @@

  static inline u64 rsvd_bits(int s, int e)
  {
+	if (unlikely(s > e))
+		return 0;
  	return ((1ULL << (e - s + 1)) - 1) << s;
  }



s == e + 1 is supported:

    (1ULL << (e - (e + 1) + 1)) - 1) << s ==

(1ULL << (e - (e + 1) + 1)) - 1) << s
	= (1ULL << (e - e - 1) + 1)) - 1) << s
	= (1ULL << (-1) + 1)) - 1) << s
	= (1ULL << (0) - 1) << s
	= (1ULL << (- 1) << s

Am I missing something?

Thanks
Tiejun

    (1ULL << 0) << s ==
    0

Is there any case where s is even bigger?

Paolo
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