Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR

Bandan Das
2015-07-12T19:12:19Z

Alex Williamson <alex.williamson@redhat.com> writes:

> On Wed, 2015-05-13 at 14:42 +0800, Xiao Guangrong wrote:
>> There are some bugs in current get_mtrr_type();
>> 1: bit 1 of mtrr_state->enabled is corresponding bit 11 of
>> IA32_MTRR_DEF_TYPE MSR which completely control MTRR's enablement
>> that means other bits are ignored if it is cleared
>>
>> 2: the fixed MTRR ranges are controlled by bit 0 of
>> mtrr_state->enabled (bit 10 of IA32_MTRR_DEF_TYPE)
>>
>> 3: if MTRR is disabled, UC is applied to all of physical memory rather
>> than mtrr_state->def_type
>>
>> Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
>> ---
>> arch/x86/kvm/mmu.c | 14 ++++++--------
>> 1 file changed, 6 insertions(+), 8 deletions(-)
>
>
> I'm seeing a significant regression in boot performance on Intel
> hardware with assigned devices that bisects back to this patch. There's
> a long delay with Seabios between the version splash and execution of
> option ROMs, and a _very_ long delay with OVMF before the display is
> initialized. The delay is long enough that users are reporting their
> previously working VM is hung with 100% CPU usage on v4.2-rc1. Thanks,
>
> Alex
>
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index b78e83f..d00cebd 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2393,19 +2393,20 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
>> static int get_mtrr_type(struct mtrr_state_type *mtrr_state,
>> u64 start, u64 end)
>> {
>> - int i;
>> u64 base, mask;
>> u8 prev_match, curr_match;
>> - int num_var_ranges = KVM_NR_VAR_MTRR;
>> + int i, num_var_ranges = KVM_NR_VAR_MTRR;
>>
>> - if (!mtrr_state->enabled)
>> - return 0xFF;
>> + /* MTRR is completely disabled, use UC for all of physical memory. */
>> + if (!(mtrr_state->enabled & 0x2))
>> + return MTRR_TYPE_UNCACHABLE;

Setting this to UC if MTRR is disabled is too restrictive maybe..

Bandan

>> /* Make end inclusive end, instead of exclusive */
>> end--;
>>
>> /* Look in fixed ranges. Just return the type as per start */
>> - if (mtrr_state->have_fixed && (start < 0x100000)) {
>> + if (mtrr_state->have_fixed && (mtrr_state->enabled & 0x1) &&
>> + (start < 0x100000)) {
>> int idx;
>>
>> if (start < 0x80000) {
>> @@ -2428,9 +2429,6 @@ static int get_mtrr_type(struct mtrr_state_type *mtrr_state,
>> * Look of multiple ranges matching this address and pick type
>> * as per MTRR precedence
>> */
>> - if (!(mtrr_state->enabled & 2))
>> - return mtrr_state->def_type;
>> -
>> prev_match = 0xFF;
>> for (i = 0; i < num_var_ranges; ++i) {
>> unsigned short start_state, end_state;
>
>
>
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