Re: [PATCH 07/15] KVM: MTRR: improve kvm_mtrr_get_guest_memory_type

[Xiao Guangrong]

On 06/01/2015 05:16 PM, Paolo Bonzini wrote:
> On 30/05/2015 12:59, Xiao Guangrong wrote:
> >   - kvm_mtrr_get_guest_memory_type() only checks one page in MTRRs so that
> >     it's unnecessary to check to see if the range is partially covered in
> >     MTRR
> >
> >   - optimize the check of overlap memory type and add some comments to explain
> >     the precedence
> >
> > Signed-off-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mtrr.c | 89 ++++++++++++++++++++++++++---------------------------
> >   1 file changed, 44 insertions(+), 45 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> > index bc9c6da..d3c06d2 100644
> > --- a/arch/x86/kvm/mtrr.c
> > +++ b/arch/x86/kvm/mtrr.c
> > @@ -231,24 +231,16 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
> >   	return 0;
> >   }
> >
> > -/*
> > - * The function is based on mtrr_type_lookup() in
> > - * arch/x86/kernel/cpu/mtrr/generic.c
> > - */
> > -static int get_mtrr_type(struct kvm_mtrr *mtrr_state,
> > -			 u64 start, u64 end)
> > +u8 kvm_mtrr_get_guest_memory_type(struct kvm_vcpu *vcpu, gfn_t gfn)
> >   {
> > -	u64 base, mask;
> > -	u8 prev_match, curr_match;
> > -	int i, num_var_ranges = KVM_NR_VAR_MTRR;
> > +	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
> > +	u64 base, mask, start = gfn_to_gpa(gfn);
> > +	int i, num_var_ranges = KVM_NR_VAR_MTRR, type_mask, type = -1;
>
> Do not mix initialized and uninitialized variables on the same line
> (preexisting, I know, but let's fix it instead of making it worse :)).
> Please put each initialized variable on a separate line.

Okay. Will follow this style in the future development.

> Also please initialize type_mask here (more on this below).
>
> >   	/* MTRR is completely disabled, use UC for all of physical memory. */
> >   	if (!mtrr_state->mtrr_enabled)
> >   		return MTRR_TYPE_UNCACHABLE;
> >
> > -	/* Make end inclusive end, instead of exclusive */
> > -	end--;
> > -
> >   	/* Look in fixed ranges. Just return the type as per start */
> >   	if (mtrr_state->fixed_mtrr_enabled && (start < 0x100000)) {
> >   		int idx;
> > @@ -273,9 +265,9 @@ static int get_mtrr_type(struct kvm_mtrr *mtrr_state,
> >   	 * Look of multiple ranges matching this address and pick type
> >   	 * as per MTRR precedence
> >   	 */
> > -	prev_match = 0xFF;
> > +	type_mask = (1 << MTRR_TYPE_WRBACK) | (1 << MTRR_TYPE_WRTHROUGH);
> >   	for (i = 0; i < num_var_ranges; ++i) {
> > -		unsigned short start_state, end_state;
> > +		int curr_type;
> >
> >   		if (!(mtrr_state->var_ranges[i].mask & (1 << 11)))
> >   			continue;
> > @@ -283,50 +275,57 @@ static int get_mtrr_type(struct kvm_mtrr *mtrr_state,
> >   		base = mtrr_state->var_ranges[i].base & PAGE_MASK;
> >   		mask = mtrr_state->var_ranges[i].mask & PAGE_MASK;
> >
> > -		start_state = ((start & mask) == (base & mask));
> > -		end_state = ((end & mask) == (base & mask));
> > -		if (start_state != end_state)
> > -			return 0xFE;
> > -
> >   		if ((start & mask) != (base & mask))
> >   			continue;
> >
> > -		curr_match = mtrr_state->var_ranges[i].base & 0xff;
> > -		if (prev_match == 0xFF) {
> > -			prev_match = curr_match;
> > +		/*
> > +		 * Please refer to Intel SDM Volume 3: 11.11.4.1 MTRR
> > +		 * Precedences.
> > +		 */
> > +
> > +		curr_type = mtrr_state->var_ranges[i].base & 0xff;
> > +		if (type == -1) {
> > +			type = curr_type;
> >   			continue;
> >   		}
> >
> > -		if (prev_match == MTRR_TYPE_UNCACHABLE ||
> > -		    curr_match == MTRR_TYPE_UNCACHABLE)
> > +		/*
> > +		 * If two or more variable memory ranges match and the
> > +		 * memory types are identical, then that memory type is
> > +		 * used.
> > +		 */
> > +		if (type == curr_type)
> > +			continue;
> > +
> > +		/*
> > +		 * If two or more variable memory ranges match and one of
> > +		 * the memory types is UC, the UC memory type used.
> > +		 */
> > +		if (curr_type == MTRR_TYPE_UNCACHABLE)
> >   			return MTRR_TYPE_UNCACHABLE;
> >
> > -		if ((prev_match == MTRR_TYPE_WRBACK &&
> > -		     curr_match == MTRR_TYPE_WRTHROUGH) ||
> > -		    (prev_match == MTRR_TYPE_WRTHROUGH &&
> > -		     curr_match == MTRR_TYPE_WRBACK)) {
> > -			prev_match = MTRR_TYPE_WRTHROUGH;
> > -			curr_match = MTRR_TYPE_WRTHROUGH;
> > +		/*
> > +		 * If two or more variable memory ranges match and the
> > +		 * memory types are WT and WB, the WT memory type is used.
> > +		 */
> > +		if (((1 << type) & type_mask) &&
> > +		      ((1 << curr_type) & type_mask)) {
>
> Please inline definition of type_mask in the "if", or rename it to
> "wt_wb_mask" and make it const.  Or another suggestion below...

Okay, it's a better name indeed.

> > +			type = MTRR_TYPE_WRTHROUGH;
> > +			continue;
> >   		}
> >
> > -		if (prev_match != curr_match)
> > -			return MTRR_TYPE_UNCACHABLE;
> > +		/*
> > +		 * For overlaps not defined by the above rules, processor
> > +		 * behavior is undefined.
> > +		 */
>
> Perhaps just use type = MIN(type, curr_type), which also happens to get
> WT vs. WB right?  You can also add a
>
> 	BUILD_BUG_ON(MTRR_TYPE_WRTHROUGH > MTRR_TYPE_WRBACK);
>
> to ensure that the WT vs. WB precedence is correct.

Only WT and WB are allowed to be overlapped here otherwise is
"undefined behavior". For example if the types are MTRR_TYPE_WRPROT
and MTRR_TYPE_WRTHROUGH, min() will get MTRR_TYPE_WRTHROUGH rather than
"undefined behavior".
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/