linux-kernel - Re: [PATCH 1/5] KVM: SVM: Move msrpm offset calculation to seperate function

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Date:	Fri, 26 Feb 2010 11:25:07 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: SVM: Move msrpm offset calculation to
 seperate function

On Fri, Feb 26, 2010 at 12:20:10PM +0200, Avi Kivity wrote:
> On 02/25/2010 07:15 PM, Joerg Roedel wrote:
> >The algorithm to find the offset in the msrpm for a given
> >msr is needed at other places too. Move that logic to its
> >own function.
> >
> >  #define MAX_INST_SIZE 15
> >
> >@@ -417,23 +439,22 @@ err_1:
> >  static void set_msr_interception(u32 *msrpm, unsigned msr,
> >  				 int read, int write)
> >  {
> >-	int i;
> >+	u8 bit_read, bit_write;
> >+	unsigned long tmp;
> >+	u32 offset;
> >
> >-	for (i = 0; i<  NUM_MSR_MAPS; i++) {
> >-		if (msr>= msrpm_ranges[i]&&
> >-		    msr<  msrpm_ranges[i] + MSRS_IN_RANGE) {
> >-			u32 msr_offset = (i * MSRS_IN_RANGE + msr -
> >-					  msrpm_ranges[i]) * 2;
> >-
> >-			u32 *base = msrpm + (msr_offset / 32);
> >-			u32 msr_shift = msr_offset % 32;
> >-			u32 mask = ((write) ? 0 : 2) | ((read) ? 0 : 1);
> >-			*base = (*base&  ~(0x3<<  msr_shift)) |
> >-				(mask<<  msr_shift);
> >-			return;
> >-		}
> >-	}
> >-	BUG();
> >+	offset    = svm_msrpm_offset(msr);
> >+	bit_read  = 2 * (msr&  0x0f);
> >+	bit_write = 2 * (msr&  0x0f) + 1;
> >+
> >+	BUG_ON(offset == MSR_INVALID);
> >+
> >+	tmp = msrpm[offset];
> >+
> >+	read  ? clear_bit(bit_read,&tmp) : set_bit(bit_read,&tmp);
> >+	write ? clear_bit(bit_write,&tmp) : set_bit(bit_write,&tmp);
> >+
> >+	msrpm[offset] = tmp;
> >  }
> 
> This can fault - set_bit() accesses an unsigned long, which can be 8
> bytes, while offset can point into the last u32 of msrpm.  So this
> needs either to revert to u32 shift/mask ops or msrpm be changed to
> a ulong array (actually better, since bitmaps in general are defined
> as arrays of ulongs).

Ah true, I will fix that. Thanks.

> btw, the op-level ternary expression is terrible, relying solely on
> *_bit()'s side effects.  Please convert to an ordinary if.
> 
> btw2, use __set_bit() which atomic operation is not needed.

Right, will switch to __set_bit and __clear_bit.

	Joerg


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