On 2017-11-08 17:36:01, Laszlo Ersek wrote:
> Hi Ray,
>
> On 10/12/17 10:48, Ruiyu Ni wrote:
> > The new algorithm converts the problem calculating optimal
> > MTRR settings (using least MTRR registers) to the problem finding
> > the shortest path in a graph.
> > The memory required in extreme but rare case can be up to 256KB,
> > so using local stack buffer is impossible considering current
> > DxeIpl only allocates 128KB stack.
> >
> > The patch changes existing MtrrSetMemoryAttributeInMtrrSettings() and
> > MtrrSetMemoryAttribute() to use the 4-page stack buffer for
> > calculation. The two APIs return BUFFER_TOO_SMALL when the buffer
> > is too small for calculation.
>
> [snip]
>
> > +#define SCRATCH_BUFFER_SIZE (4 * SIZE_4KB)
>
> [snip]
>
> > RETURN_STATUS
> > EFIAPI
> > -MtrrSetMemoryAttribute (
> > +MtrrSetMemoryAttributeInMtrrSettings (
> > + IN OUT MTRR_SETTINGS *MtrrSetting,
> > IN PHYSICAL_ADDRESS BaseAddress,
> > IN UINT64 Length,
> > IN MTRR_MEMORY_CACHE_TYPE Attribute
> > )
> > {
> > RETURN_STATUS Status;
> > + UINT8 Scratch[SCRATCH_BUFFER_SIZE];
>
> [snip]
>
> (This patch is now commit 2bbd7e2fbd4b.)
>
> Today I managed to spend time on
>
> https://bugzilla.tianocore.org/show_bug.cgi?id=747
>
> (which is in turn based on the earlier mailing list thread
>
> [edk2] dynamic PCD impact on temporary PEI memory
> https://lists.01.org/pipermail/edk2-devel/2017-October/016213.html
> ).
>
> While writing the patches, I found the root cause of BZ#747:
> "OvmfPkg/PlatformPei" calls MtrrLib APIs, and due to the above 16KB
> stack allocation, MtrrLib overflow's OVMF's 16KB (total) temp SEC/PEI
> stack.
I thought it was considered bad form to use a significant portion of
the stack (> ~100 bytes) via local variables. This used to
occasionally break MSVC builds as MS would insert a stack check call
if the locals size exceeded some threshold.
For a BASE library, I think this should go beyond "bad form" and into
not allowed.
-Jordan
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