linux-kernel - Re: [RFC PATCH v2 28/69] KVM: Add per-VM flag to mark read-only memory as unsupported

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Message-ID: <1a8bfc55-e0f2-dbba-c5a7-48e648c376ef@amd.com>
Date:   Tue, 6 Jul 2021 14:04:03 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>, isaku.yamahata@...el.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     brijesh.singh@....com, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC PATCH v2 28/69] KVM: Add per-VM flag to mark read-only
 memory as unsupported



On 7/6/21 9:03 AM, Paolo Bonzini wrote:
> On 03/07/21 00:04, isaku.yamahata@...el.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@...el.com>
>>
>> Add a flag for TDX to flag RO memory as unsupported and propagate it to
>> KVM_MEM_READONLY to allow reporting RO memory as unsupported on a per-VM
>> basis.  TDX1 doesn't expose permission bits to the VMM in the SEPT
>> tables, i.e. doesn't support read-only private memory.
>>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
>> Co-developed-by: Sean Christopherson <sean.j.christopherson@...el.com>
>> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
>> ---
>>   arch/x86/kvm/x86.c       | 4 +++-
>>   include/linux/kvm_host.h | 4 ++++
>>   virt/kvm/kvm_main.c      | 8 +++++---
>>   3 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index cd9407982366..87212d7563ae 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3897,7 +3897,6 @@ int kvm_vm_ioctl_check_extension(struct kvm 
>> *kvm, long ext)
>>       case KVM_CAP_ASYNC_PF_INT:
>>       case KVM_CAP_GET_TSC_KHZ:
>>       case KVM_CAP_KVMCLOCK_CTRL:
>> -    case KVM_CAP_READONLY_MEM:
>>       case KVM_CAP_HYPERV_TIME:
>>       case KVM_CAP_IOAPIC_POLARITY_IGNORED:
>>       case KVM_CAP_TSC_DEADLINE_TIMER:
>> @@ -4009,6 +4008,9 @@ int kvm_vm_ioctl_check_extension(struct kvm 
>> *kvm, long ext)
>>           if (static_call(kvm_x86_is_vm_type_supported)(KVM_X86_TDX_VM))
>>               r |= BIT(KVM_X86_TDX_VM);
>>           break;
>> +    case KVM_CAP_READONLY_MEM:
>> +        r = kvm && kvm->readonly_mem_unsupported ? 0 : 1;
>> +        break;
>>       default:
>>           break;
>>       }
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index ddd4d0f68cdf..7ee7104b4b59 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -597,6 +597,10 @@ struct kvm {
>>       unsigned int max_halt_poll_ns;
>>       u32 dirty_ring_size;
>> +#ifdef __KVM_HAVE_READONLY_MEM
>> +    bool readonly_mem_unsupported;
>> +#endif
>> +
>>       bool vm_bugged;
>>   };
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 52d40ea75749..63d0c2833913 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1258,12 +1258,14 @@ static void update_memslots(struct 
>> kvm_memslots *slots,
>>       }
>>   }
>> -static int check_memory_region_flags(const struct 
>> kvm_userspace_memory_region *mem)
>> +static int check_memory_region_flags(struct kvm *kvm,
>> +                     const struct kvm_userspace_memory_region *mem)
>>   {
>>       u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
>>   #ifdef __KVM_HAVE_READONLY_MEM
>> -    valid_flags |= KVM_MEM_READONLY;
>> +    if (!kvm->readonly_mem_unsupported)
>> +        valid_flags |= KVM_MEM_READONLY;
>>   #endif
>>       if (mem->flags & ~valid_flags)
>> @@ -1436,7 +1438,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>>       int as_id, id;
>>       int r;
>> -    r = check_memory_region_flags(mem);
>> +    r = check_memory_region_flags(kvm, mem);
>>       if (r)
>>           return r;
>>
> 
> For all these flags, which of these limitations will be common to SEV-ES 
> and SEV-SNP (ExtINT injection, MCE injection, changing TSC, read-only 
> memory, dirty logging)?  Would it make sense to use vm_type instead of 
> all of them?  I guess this also guides the choice of whether to use a 
> single vm-type for TDX and SEV-SNP or two.  Probably two is better, and 
> there can be static inline bool functions to derive the support flags 
> from the vm-type.
> 

The SEV-ES does not need any of these flags. However, with SEV-SNP, we 
may able use the ExtINT injection.

-Brijesh