Messages in this thread | | | Subject | Re: [RFC Part1 PATCH v3 16/17] X86/KVM: Provide support to create Guest and HV shared per-CPU variables | From | Brijesh Singh <> | Date | Wed, 30 Aug 2017 11:18:42 -0500 |
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Hi Boris,
On 08/29/2017 05:22 AM, Borislav Petkov wrote:
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> On Mon, Jul 24, 2017 at 02:07:56PM -0500, Brijesh Singh wrote: >> Some KVM specific MSR's (steal-time, asyncpf, avic_eio) allocates per-CPU > > MSRs > >> variable at compile time and share its physical address with hypervisor. > > That sentence needs changing - the MSRs don't allocate - for them gets > allocated. > >> It presents a challege when SEV is active in guest OS, when SEV is active, >> the guest memory is encrypted with guest key hence hypervisor will not >> able to modify the guest memory. When SEV is active, we need to clear the >> encryption attribute (aka C-bit) of shared physical addresses so that both >> guest and hypervisor can access the data. > > This whole paragraph needs rewriting. >
I will improve the commit message in next rev.
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>> +/* NOTE: function is marked as __ref because it is used by __init functions */ > > No need for that comment. > > What should you look into is why do you need to call the early versions: > > " * producing a warning (of course, no warning does not mean code is > * correct, so optimally document why the __ref is needed and why it's OK)." > > And we do have the normal set_memory_decrypted() etc helpers so why > aren't we using those? >
Since kvm_guest_init() is called early in the boot process hence we will not able to use set_memory_decrypted() function. IIRC, if we try calling set_memory_decrypted() early then we will hit a BUG_ON [1] -- mainly when it tries to flush the caches.
[1] http://elixir.free-electrons.com/linux/latest/source/arch/x86/mm/pageattr.c#L167
> If you need to use the early ones too, then you probably need to > differentiate this in the callers by passing a "bool early", which calls > the proper flavor. >
Sure I can rearrange code to make it more readable and use "bool early" parameter to differentiate it.
>> +static int __ref kvm_map_hv_shared_decrypted(void) >> +{ >> + static int once, ret; >> + int cpu; >> + >> + if (once) >> + return ret; > > So this function gets called per-CPU but you need to do this ugly "once" > thing - i.e., global function called in a per-CPU context. > > Why can't you do that mapping only on the current CPU and then > when that function is called on the next CPU, it will do the same thing > on that next CPU? >
Yes, it can be done but I remember running into issues during the CPU hot plug. The patch uses early_set_memory_decrypted() -- which calls kernel_physical_mapping_init() to split the large pages into smaller. IIRC, the API did not work after the system is successfully booted. After the system is booted we must use the set_memory_decrypted().
I was trying to avoid mixing early and no-early set_memory_decrypted() but if feedback is: use early_set_memory_decrypted() only if its required otherwise use set_memory_decrypted() then I can improve the logic in next rev. thanks
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>> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h >> index da0be9a..52854cf 100644 >> --- a/include/asm-generic/vmlinux.lds.h >> +++ b/include/asm-generic/vmlinux.lds.h >> @@ -783,6 +783,9 @@ >> . = ALIGN(cacheline); \ >> *(.data..percpu) \ >> *(.data..percpu..shared_aligned) \ >> + . = ALIGN(PAGE_SIZE); \ >> + *(.data..percpu..hv_shared) \ >> + . = ALIGN(PAGE_SIZE); \ >> VMLINUX_SYMBOL(__per_cpu_end) = .; > > Yeah, no, you can't do that. That's adding this section unconditionally > on *every* arch. You need to do some ifdeffery like it is done at the > beginning of that file and have this only on the arch which supports SEV. >
Will do . thanks
-Brijesh
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