linux-kernel - Re: [PATCH V2 05/12] KVM: TDX: Implement TDX vcpu enter/exit path

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Message-ID: <3e64b29e-34eb-4f9e-b7d1-a7803665ca55@intel.com>
Date: Thu, 27 Feb 2025 20:37:28 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>, <pbonzini@...hat.com>,
	<seanjc@...gle.com>
CC: <kvm@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
	<kai.huang@...el.com>, <reinette.chatre@...el.com>,
	<tony.lindgren@...ux.intel.com>, <binbin.wu@...ux.intel.com>,
	<dmatlack@...gle.com>, <isaku.yamahata@...el.com>, <nik.borisov@...e.com>,
	<linux-kernel@...r.kernel.org>, <yan.y.zhao@...el.com>, <chao.gao@...el.com>,
	<weijiang.yang@...el.com>
Subject: Re: [PATCH V2 05/12] KVM: TDX: Implement TDX vcpu enter/exit path

On 25/02/25 08:15, Xiaoyao Li wrote:
> On 2/24/2025 8:27 PM, Adrian Hunter wrote:
>> On 20/02/25 15:16, Xiaoyao Li wrote:
>>> On 1/29/2025 5:58 PM, Adrian Hunter wrote:
>>>> +#define TDX_REGS_UNSUPPORTED_SET    (BIT(VCPU_EXREG_RFLAGS) |    \
>>>> +                     BIT(VCPU_EXREG_SEGMENTS))
>>>> +
>>>> +fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>>>> +{
>>>> +    /*
>>>> +     * force_immediate_exit requires vCPU entering for events injection with
>>>> +     * an immediately exit followed. But The TDX module doesn't guarantee
>>>> +     * entry, it's already possible for KVM to_think_ it completely entry
>>>> +     * to the guest without actually having done so.
>>>> +     * Since KVM never needs to force an immediate exit for TDX, and can't
>>>> +     * do direct injection, just warn on force_immediate_exit.
>>>> +     */
>>>> +    WARN_ON_ONCE(force_immediate_exit);
>>>> +
>>>> +    trace_kvm_entry(vcpu, force_immediate_exit);
>>>> +
>>>> +    tdx_vcpu_enter_exit(vcpu);
>>>> +
>>>> +    vcpu->arch.regs_avail &= ~TDX_REGS_UNSUPPORTED_SET;
>>>
>>> I don't understand this. Why only clear RFLAGS and SEGMENTS?
>>>
>>> When creating the vcpu, vcpu->arch.regs_avail = ~0 in kvm_arch_vcpu_create().
>>>
>>> now it only clears RFLAGS and SEGMENTS for TDX vcpu, which leaves other bits set. But I don't see any code that syncs the guest value of into vcpu->arch.regs[reg].
>>
>> TDX guest registers are generally not known but
>> values are placed into vcpu->arch.regs when needed
>> to work with common code.
>>
>> We used to use ~VMX_REGS_LAZY_LOAD_SET and tdx_cache_reg()
>> which has since been removed.
>>
>> tdx_cache_reg() did not support RFLAGS, SEGMENTS,
>> EXIT_INFO_1/EXIT_INFO_2 but EXIT_INFO_1/EXIT_INFO_2 became
>> needed, so that just left RFLAGS, SEGMENTS.
> 
> Quote what Sean said [1]
> 
>   “I'm also not convinced letting KVM read garbage for RIP, RSP, CR3, or
>   PDPTRs is at all reasonable.  CR3 and PDPTRs should be unreachable,
>   and I gotta imagine the same holds true for RSP.  Allow reads/writes
>   to RIP is fine, in that it probably simplifies the overall code.”
> 
> We need to justify why to let KVM read "garbage" of VCPU_REGS_RIP,
> VCPU_EXREG_PDPTR, VCPU_EXREG_CR0, VCPU_EXREG_CR3, VCPU_EXREG_CR4,
> VCPU_EXREG_EXIT_INFO_1, and VCPU_EXREG_EXIT_INFO_2 are neeed.
> 
> The changelog justify nothing for it.

Could add VCPU_REGS_RIP, VCPU_REGS_RSP, VCPU_EXREG_CR3, VCPU_EXREG_PDPTR.
But not VCPU_EXREG_CR0 nor VCPU_EXREG_CR4 since we started using them.

> 
> btw, how EXIT_INFO_1/EXIT_INFO_2 became needed? It seems I cannot find any TDX code use them.

vmx_get_exit_qual() / vmx_get_intr_info() are now used by TDX.

> 
> [1] https://lore.kernel.org/all/Z2GiQS_RmYeHU09L@google.com/
> 
>>>
>>>> +    trace_kvm_exit(vcpu, KVM_ISA_VMX);
>>>> +
>>>> +    return EXIT_FASTPATH_NONE;
>>>> +}
>>>
>>
>