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Date: Wed, 03 Jun 2015 10:12:00 +0800
From: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/15] KVM: MTRR: improve kvm_mtrr_get_guest_memory_type
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@...ux.intel.com>
>> ---
>> 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".
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