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

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Message-ID: <d9924ccd-7322-48aa-93be-82620f72791c@intel.com>
Date: Tue, 25 Feb 2025 14:15:12 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Adrian Hunter <adrian.hunter@...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 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.

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

[1] https://lore.kernel.org/all/Z2GiQS_RmYeHU09L@google.com/

>>
>>> +    trace_kvm_exit(vcpu, KVM_ISA_VMX);
>>> +
>>> +    return EXIT_FASTPATH_NONE;
>>> +}
>>
>