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

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Message-ID: <d3f83243-5526-4196-956c-de349bebd81f@redhat.com>
Date: Thu, 6 Mar 2025 19:19:07 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Adrian Hunter <adrian.hunter@...el.com>, Xiaoyao Li
 <xiaoyao.li@...el.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 2/27/25 19:37, Adrian Hunter wrote:
> 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.

Hi Adrian,

how is CR0 used? And CR4 is only used other than for loading the XSAVE 
state, I think?

I will change this to a list of specific available registers instead of 
using "&= ~", and it would be even better if CR0/CR4 are not on the list.

Paolo

>> 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;
>>>>> +}
>>>>
>>>
>>
> 
>