linux-kernel - Re: [PATCH] selftests: kvm: fix overlapping addresses in memslot_perf

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Date:   Sat, 29 May 2021 12:20:16 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] selftests: kvm: fix overlapping addresses in
 memslot_perf_test

On 28/05/21 21:51, Maciej S. Szmigiero wrote:
> On 28.05.2021 21:11, Paolo Bonzini wrote:
>> The memory that is allocated in vm_create is already mapped close to
>> GPA 0, because test_execute passes the requested memory to
>> prepare_vm.  This causes overlapping memory regions and the
>> test crashes.  For simplicity just move MEM_GPA higher.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> I am not sure that I understand the issue correctly, is vm_create_default()
> already reserving low GPAs (around 0x10000000) on some arches or run
> environments?

It maps the number of pages you pass in the second argument, see
vm_create.

   if (phy_pages != 0)
     vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
                                 0, 0, phy_pages, 0);

In this case:

   data->vm = vm_create_default(VCPU_ID, mempages, guest_code);

called here:

   if (!prepare_vm(data, nslots, maxslots, tdata->guest_code,
                   mem_size, slot_runtime)) {

where mempages is mem_size, which is declared as:

         uint64_t mem_size = tdata->mem_size ? : MEM_SIZE_PAGES;

but actually a better fix is just to pass a small fixed value (e.g. 
1024) to vm_create_default, since all other regions are added by hand.

Paolo