Messages in this thread Patch in this message | | | Subject | Re: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM. | From | Yu Zhang <> | Date | Mon, 21 Aug 2017 15:27:51 +0800 |
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On 8/18/2017 8:50 PM, Paolo Bonzini wrote: > On 18/08/2017 10:28, Yu Zhang wrote: >> >> On 8/17/2017 10:29 PM, Paolo Bonzini wrote: >>> On 17/08/2017 13:53, Yu Zhang wrote: >>>> On 8/17/2017 7:57 PM, Paolo Bonzini wrote: >>>>> On 12/08/2017 15:35, Yu Zhang wrote: >>>>>> index a98b88a..50107ae 100644 >>>>>> --- a/arch/x86/kvm/emulate.c >>>>>> +++ b/arch/x86/kvm/emulate.c >>>>>> @@ -694,7 +694,7 @@ static __always_inline int __linearize(struct >>>>>> x86_emulate_ctxt *ctxt, >>>>>> switch (mode) { >>>>>> case X86EMUL_MODE_PROT64: >>>>>> *linear = la; >>>>>> - if (is_noncanonical_address(la)) >>>>>> + if (emul_is_noncanonical_address(la, ctxt)) >>>>>> goto bad; >>>>>> *max_size = min_t(u64, ~0u, (1ull << 48) - la); >>>>> Oops, you missed one here. Probably best to use ctxt_virt_addr_bits >>>>> and >>>>> then "inline" emul_is_noncanonical_address as "get_canonical(la, >>>>> va_bits) != la". >>>> Sorry, I just sent out the v2 patch set without noticing this reply. :-) >>>> >>>> The emul_is_noncanonical() is defined in x86.h so that no >>>> ctxt_virt_addr_bits needed in emulate.c, are you >>>> suggesting to use ctx_virt_addr_bits in this file each time before >>>> emul_is_noncanonical_address() is called? >>> No, only in this instance which uses "48" after the call to >>> emul_is_noncanonical_address. >> Sorry, Paolo. I still do not quite get it. >> Do you mean the >> *max_size = min_t(u64, ~0u, (1ull << 48) - la); >> also need to be changed? >> >> But I do not understand why this statement is used like this. My >> understanding is that >> for 64 bit scenario, the *max_size is calculated to guarantee la + >> *max_size still falls in >> the canonical address space. >> >> And if above understanding is correct, I think it should be something >> like below: >> *max_size = min_t(u64, ~0u - la, (1ull << 48) - la); > The "~0u" part is simply because max_size has 32-bit size (it's an > unsigned int variable), while (1ull << 48) - la has 64-bit size. It > protects from the overflow.
Oh, right. "~0u" is only an unsigned int. Thanks for your clarification. :-)
But what if value of "la" falls in between 0xFFFFFFFFFFFFFFFF and 0xFFFF000000000000? (1ull << 48) - la may result in something between 0x1000000000001 and 0x2000000000000, and the *max_size would be 4G - 1 in this scenario. For instance, when "la" is 0xFFFFFFFFFFFFFFF0(unlikely in practice though), the *max_size we are expecting should be 15, instead of 4G - 1.
If above understanding is correct, maybe we should change this code as below: @@ -690,16 +690,21 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt, ulong la; u32 lim; u16 sel; + u64 canonical_limit; + u8 va_bits;
la = seg_base(ctxt, addr.seg) + addr.ea; *max_size = 0; switch (mode) { case X86EMUL_MODE_PROT64: *linear = la; - if (emul_is_noncanonical_address(la, ctxt)) + va_bits = ctxt_virt_addr_bits(ctxt); + if (get_canonical(la, va_bits) != la) goto bad;
- *max_size = min_t(u64, ~0u, (1ull << 48) - la); + canonical_limit = (la & (1 << va_bits)) ? + ~0ull : ((1 << va_bits) -1); + *max_size = min_t(u64, ~0u, canonical_limit - la + 1);
Does this sound reasonable? BTW, I did not use min_t(u64, ~0ull - la + 1, (1 << va_bits) - la) here, because I still would like to keep *max_size as an unsigned int, and my previous suggestion may cause the return value of min_t be truncated.
Yu
>> And with LA57, may better be changed to: >> *max_size = min_t(u64, ~0u - la, (1ull << ctxt_virt_addr_bits(ctxt)) - >> la); >> And for the above >> if (emul_is_noncanonical_address(la, ctxt)) >> we may just leave it as it is. > Yes, exactly. But since emul_is_noncanonical_address is already using > ctxt_virt_addr_bits(ctxt), it may make sense to compute > ctxt_virt_addr_bits(ctxt) once and then reuse it twice, once in > get_canonical(la, va_bits) != la and once in (1ull << va_bits) - la. > > Paolo > >> Is this understanding correct? Or did I misunderstand your comments? :-) >> >> Thanks >> Yu >>> Paolo >>> >
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