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

[Alex Williamson]

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@xxxxxxxxxxxxxxx>
> ---
>  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;
>  
>  	/* 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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