Re: input/output error _at_boot

> On 9. märts 2017, at 15:03, Dexuan Cui <decui_at_microsoft.com> wrote:
> 
>> From: owner-freebsd-current_at_freebsd.org [mailto:owner-freebsd-
>> current_at_freebsd.org] On Behalf Of Toomas Soome
>> 
>> IMO there are multiple issues around this problem and workaround.
>> 
>> First of all, to control UEFI memory allocation, the AllocatePages() has options:
>> 
>> AllocateAnyPages,
>> AllocateMaxAddress,
>> AllocateAddress
>> 
>> On x86, we use:
>> 
>>        staging = 1024*1024*1024;
>>        status = BS->AllocatePages(AllocateMaxAddress, EfiLoaderData,
>>            nr_pages, &staging);
>> 
>> Which means:
>> 
>> "Allocation requests of Type AllocateMaxAddress allocate any available range of
>> pages whose uppermost address is less than or equal to the address pointed to
>> by Memory on input.”
>> 
>> So, we are asking for an amount of memory (64MB), with condition that all the
>> pages should be below 1GB.
>> 
>> And we get it. If hyper-v is in fact returning us memory from already occupied
>> area - there can be exactly one conclusion - it is bug in hyper-v.
> 
> Hyper-V has no bug here: Hyper-V doesn't return memory from already occupied
> area. The issue is: the loader here tries to write the 64MB staging area (BTW, it's
> 48MB in 10.3) into the physical memory range [2MB, 2MB+64MB) -- the loader
> assumes this range is writable. However, this is not true with Hyper-V EFI
> firmware: there is a read-only BootServicesData memory block starting at
> about 47.449MB, causing a crash in the loader.
> 
> If you're interested, the whole long story is in the below link.  :-)
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211746, e.g. please see the
> screenshot in comment #8.
> 


ah, right, so it already does the relocation and will get busted there, sorry, missed that:D


> 
>> Note, this allocation method does *not* set the starting point for allocation, it
>> can return us *any* chunk of memory of given size, below 1GB.
> Yes. This can potentially cause new issues...
> 
>> So the attempt to control such allocation by size, is unfortunately flawed - it
>> really does not control the allocation.
> Yes, you're correct.
> The patch is flawed. I only expect (or hope) it can work around the issues with
> typical Hyper-V UEFI firmware.
> In my test, it works with Hyper-V 2012 R2 and 2016.
> I hope it could work in future Hyper-V too...
> 
>> Note that I have also seen AllocateAddress failures - there was nicely available
>> chunk of memory, but the firmware just did not allocate with given address (it
>> did happen with OVMF + qemu).
>> 
>> The secondary flaw there is also about firmware. Sure, with UEFI you can have
>> “random” allocations and the actual control over memory is actually problem,
>> but to plant an “egg”  in 1MB-1GB range, where you have most chances any OS
>> will live - IMO this is just stupid.
>> 
>> The only real solution here is to either rise the MaxAddress limit or use
>> AllocateAnyPages, get kernel loaded into the memory, and after switching off
>> the boot services and before jumping to kernel, relocate the kernel to available
>> location below 1GB…
> Yes. IMO the biggest issue is that currently the kernel can't be relocated... :-(
> It's a long term work to make it relocatable, I'm afraid.
> 
> Thanks,
> -- Dexuan

true, and there are other systems with same issue. relocatable kernels are not really that common even today;) anyhow, good work from your side;)

rgds,
toomas