linux-kernel - Re: [PATCH v4 26/28] KVM: TDX: Guard VM state transitions with "all" the locks

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Message-ID: <aQgHOcy28pSEfTZK@yzhao56-desk.sh.intel.com>
Date: Mon, 3 Nov 2025 09:36:57 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan
	<maddy@...ux.ibm.com>, Anup Patel <anup@...infault.org>, Paul Walmsley
	<pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
	<aou@...s.berkeley.edu>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda
	<imbrenda@...ux.ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, "Kirill A.
 Shutemov" <kas@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<kvmarm@...ts.linux.dev>, <kvm@...r.kernel.org>, <loongarch@...ts.linux.dev>,
	<linux-mips@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<kvm-riscv@...ts.infradead.org>, <linux-riscv@...ts.infradead.org>,
	<x86@...nel.org>, <linux-coco@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, Kai Huang
	<kai.huang@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>, Michael Roth
	<michael.roth@....com>, Vishal Annapurve <vannapurve@...gle.com>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>, Ackerley Tng
	<ackerleytng@...gle.com>
Subject: Re: [PATCH v4 26/28] KVM: TDX: Guard VM state transitions with "all"
 the locks

On Fri, Oct 31, 2025 at 10:34:51AM -0700, Sean Christopherson wrote:
> On Fri, Oct 31, 2025, Yan Zhao wrote:
> > On Thu, Oct 30, 2025 at 01:09:49PM -0700, Sean Christopherson wrote:
> > > Acquire kvm->lock, kvm->slots_lock, and all vcpu->mutex locks when
> > > servicing ioctls that (a) transition the TD to a new state, i.e. when
> > > doing INIT or FINALIZE or (b) are only valid if the TD is in a specific
> > > state, i.e. when initializing a vCPU or memory region.  Acquiring "all"
> > > the locks fixes several KVM_BUG_ON() situations where a SEAMCALL can fail
> > > due to racing actions, e.g. if tdh_vp_create() contends with either
> > > tdh_mr_extend() or tdh_mr_finalize().
> > > 
> > > For all intents and purposes, the paths in question are fully serialized,
> > > i.e. there's no reason to try and allow anything remotely interesting to
> > > happen.  Smack 'em with a big hammer instead of trying to be "nice".
> > > 
> > > Acquire kvm->lock to prevent VM-wide things from happening, slots_lock to
> > > prevent kvm_mmu_zap_all_fast(), and _all_ vCPU mutexes to prevent vCPUs
> > s/kvm_mmu_zap_all_fast/kvm_mmu_zap_memslot
> 
> Argh!  Third time's a charm?  Hopefully...
> 
> > > @@ -3170,7 +3208,8 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
> > >  
> > >  int tdx_vcpu_unlocked_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
> > >  {
> > > -	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> > > +	struct kvm *kvm = vcpu->kvm;
> > > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > reverse xmas tree ?
> 
> No, because the shorter line generates an input to the longer line.  E.g. we could
> do this if we really, really want an xmas tree:
> 
> 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> 	struct kvm *kvm = vcpu->kvm;
> 
> but this won't compile
> 
> 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> 	struct kvm *kvm = vcpu->kvm;
Ah! Sorry. My attention was caught by the line length, completely missing the
dependency :(