Re: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM.

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 





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






[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux