linux-kernel - Re: [RFC PATCH 2/2] KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal

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Message-ID: <Z2IJP-T8NVsanjNd@google.com>
Date: Tue, 17 Dec 2024 15:29:03 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, dave.hansen@...ux.intel.com, 
	rick.p.edgecombe@...el.com, kai.huang@...el.com, adrian.hunter@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...el.com, tony.lindgren@...el.com, 
	binbin.wu@...ux.intel.com, dmatlack@...gle.com, isaku.yamahata@...el.com, 
	isaku.yamahata@...il.com, nik.borisov@...e.com, linux-kernel@...r.kernel.org, 
	x86@...nel.org
Subject: Re: [RFC PATCH 2/2] KVM: TDX: Kick off vCPUs when SEAMCALL is busy
 during TD page removal

On Thu, Nov 21, 2024, Yan Zhao wrote:
> For tdh_mem_range_block(), tdh_mem_track(), tdh_mem_page_remove(),
> 
> - Upon detection of TDX_OPERAND_BUSY, retry each SEAMCALL only once.
> - During the retry, kick off all vCPUs and prevent any vCPU from entering
>   to avoid potential contentions.
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/vmx/tdx.c          | 49 +++++++++++++++++++++++++--------
>  2 files changed, 40 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 521c7cf725bc..bb7592110337 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -123,6 +123,8 @@
>  #define KVM_REQ_HV_TLB_FLUSH \
>  	KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
>  #define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE	KVM_ARCH_REQ(34)
> +#define KVM_REQ_NO_VCPU_ENTER_INPROGRESS \
> +	KVM_ARCH_REQ_FLAGS(33, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
>  
>  #define CR0_RESERVED_BITS                                               \
>  	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 60d9e9d050ad..ed6b41bbcec6 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -311,6 +311,20 @@ static void tdx_clear_page(unsigned long page_pa)
>  	__mb();
>  }
>  
> +static void tdx_no_vcpus_enter_start(struct kvm *kvm)
> +{
> +	kvm_make_all_cpus_request(kvm, KVM_REQ_NO_VCPU_ENTER_INPROGRESS);

I vote for making this a common request with a more succient name, e.g. KVM_REQ_PAUSE.
And with appropriate helpers in common code.  I could have sworn I floated this
idea in the past for something else, but apparently not.  The only thing I can
find is an old arm64 version for pausing vCPUs to emulated.  Hmm, maybe I was
thinking of KVM_REQ_OUTSIDE_GUEST_MODE?

Anyways, I don't see any reason to make this an arch specific request.