linux-kernel - Re: [PATCH v19 120/130] KVM: TDX: Add a method to ignore dirty logging

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Date: Fri, 22 Mar 2024 15:57:36 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"Zhang, Tina" <tina.zhang@...el.com>,
	"Yuan, Hang" <hang.yuan@...el.com>,
	"Huang, Kai" <kai.huang@...el.com>, "Chen, Bo2" <chen.bo@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Aktas, Erdem" <erdemaktas@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH v19 120/130] KVM: TDX: Add a method to ignore dirty
 logging

On Mon, Mar 18, 2024 at 04:16:56PM -0700,
Isaku Yamahata <isaku.yamahata@...el.com> wrote:

> On Mon, Mar 18, 2024 at 05:43:33PM +0000,
> "Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:
> 
> > On Mon, 2024-03-18 at 10:12 -0700, Isaku Yamahata wrote:
> > > I categorize as follows. Unless otherwise, I'll update this series.
> > > 
> > > - dirty log check
> > >   As we will drop this ptach, we'll have no call site.
> > > 
> > > - KVM_BUG_ON() in main.c
> > >   We should drop them because their logic isn't complex.
> > What about "KVM: TDX: Add methods to ignore guest instruction
> > emulation"? Is it cleanly blocked somehow?
> 
> KVM fault handler, kvm_mmu_page_fault(), is the caller into the emulation,
> It should skip the emulation.
> 
> As the second guard, x86_emulate_instruction(), calls
> check_emulate_instruction() callback to check if the emulation can/should be
> done.  TDX callback can return it as X86EMUL_UNHANDLEABLE.  Then, the flow goes
> to user space as error.  I'll update the vt_check_emulate_instruction().

Oops. It was wrong. It should be X86EMUL_RETRY_INSTR.  RETRY_INSTR means, let
vcpu execute the intrusion again, UNHANDLEABLE means, emulator can't emulate,
inject exception or give up with KVM_EXIT_INTERNAL_ERROR.

For TDX, we'd like to inject #VE to the guest so that the guest #VE handler
can issue TDG.VP.VMCALL<MMIO>.  The default non-present sept value has
#VE suppress bit set.  As first step, EPT violation occurs. then KVM sets
up mmio_spte with #VE suppress bit cleared. Then X86EMUL_RETRY_INSTR tells
kvm to resume vcpu to inject #VE.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>