linux-kernel - Re: [PATCH v12 13/46] KVM: x86: Prepare kvm_hv_flush

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Date:   Thu, 27 Oct 2022 10:18:14 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 13/46] KVM: x86: Prepare kvm_hv_flush_tlb() to
 handle L2's GPAs

Sean Christopherson <seanjc@...gle.com> writes:

> On Fri, Oct 21, 2022, Vitaly Kuznetsov wrote:
>> To handle L2 TLB flush requests, KVM needs to translate the specified
>> L2 GPA to L1 GPA to read hypercall arguments from there.
>> 
>> No functional change as KVM doesn't handle VMCALL/VMMCALL from L2 yet.
>> 
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/hyperv.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>> 
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index fca9c51891f5..df1efb821eb0 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -23,6 +23,7 @@
>>  #include "ioapic.h"
>>  #include "cpuid.h"
>>  #include "hyperv.h"
>> +#include "mmu.h"
>>  #include "xen.h"
>>  
>>  #include <linux/cpu.h>
>> @@ -1908,6 +1909,12 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>>  	 */
>>  	BUILD_BUG_ON(KVM_HV_MAX_SPARSE_VCPU_SET_BITS > 64);
>>  
>> +	if (!hc->fast && is_guest_mode(vcpu)) {
>
> Please add a comment explaining why only "slow" hypercalls need to translate the
> GPA from L2=>L1.
>
> With a comment (and assuming this isn't a bug),

This is intended,

For "slow" hypercalls 'hc->ingpa' is the GPA (or an 'nGPA' -- thus the
patch) in guest memory where hypercall parameters are placed, kvm reads
them with kvm_read_guest() later. For "fast" hypercalls 'ingpa' is a
misnomer as it is not an address but the first parameter (in the 'tlb
flush' case it's 'address space id' which we currently don't
analyze). We may want to add a union in 'struct kvm_hv_hcall' to make
this explicit.

The comment I'm thinking of would be:

"
/*
 * 'Slow' hypercall's first parameter is the address in guest's memory where
 * hypercall parameters are placed. This is either a GPA or a nested GPA when
 * KVM is handling the call from L2 ('direct' TLB flush), translate the address
 * here so the memory can be uniformly read with kvm_read_guest().
 */
"

>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
>

-- 
Vitaly