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

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Message-ID: <632ea548-0e64-4a62-8126-120e42f4cd64@intel.com>
Date: Mon, 24 Feb 2025 14:27:25 +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 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.

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