Re: Subject: [RFC PATCH] kvm/x86: Fix 'lpages' kvm stat for TDM MMU

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 29, 2021, Shahin, Md Shahadat Hossain wrote:
> Large pages not being created properly may result in increased memory
> access time. The 'lpages' kvm stat used to keep track of the current
> number of large pages in the system, but with TDP MMU enabled the stat
> is not showing the correct number.
> 
> This patch extends the lpages counter to cover the TDP case.
> 
> Signed-off-by: Md Shahadat Hossain Shahin <shahinmd@amazon.de>
> Cc: Bartosz Szczepanek <bsz@amazon.de>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 34207b874886..1e2a3cb33568 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -425,6 +425,12 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  
>  	if (old_spte == new_spte)
>  		return;
> +	
> +	if (is_large_pte(old_spte))
> +		--kvm->stat.lpages;
> +	
> +	if (is_large_pte(new_spte))
> +		++kvm->stat.lpages;

Hrm, kvm->stat.lpages could get corrupted when __handle_changed_spte() is called
under read lock, e.g. if multiple vCPUs are faulting in memory.